I had the same with key 2. When looking very good at it in bright light, I could see a tiny (like a hair) piece of plastic (I think itās called a āburrā in English) at the spot where the key was stuck (top left in my case). This piece was attached to the key.
I guess it was a tiny piece of waste from cutting the buttons. I very carefully removed it with a razor. After doing so, it was solved.
Hope this helps.
It will go away!! Trust me! Play it some more and it will losen again!
Cheers
Nr 9 for me. Hope it fixes it self. Bit annoying - otherwise I really enjoy the digi.
I had several buttons that got stuck when I first got it and was really fucking worried. after playing with the unit extensively for a few weeks the buttons quit sticking and theyāre effinā great now! I love the buttons on this thing so much! hate the midi freezes and no way to backup the unit.
Also here to say, it will go away eventually! I had a sticky tr1 and func button (SUPER ANNOYING). After a few weeks it went away for good.
FML!
Fresh out of the box, all of my keys had a slight tendency to stick. A few days later and Iām ready to throw the thing out of the window, button 3 makes me want to end it all.
WHY ELEKTRON?! WHY YOU DO THIS!?
As a side note, my screen cover wasnāt properly attached either (seen one other person mention that) I had to push the plastic back down onto the bezel, so there are QC issues as well as what appears to be a horrible design flaw. If this is the price of making a ālower costā machine, just stop already. 
EDIT: I forgot to mention the encoders that vary in tightness as you rotate them (as if they are bent in some way) that said, it was actually a major issue on my A4 as well. All of that plus an unusable OP-1 and my faith in Swedish engineering has just hit an all time low. I wonder if Nord is as bad?
I still have (after all those months) a sticky button 13ā¦
number 7 for me
12/13/14
Had the same problem with my A4 MK2, to fix the malfunction I kept returning it to the retailer until I got a perfect unitā¦ I keep reading about people here excepting malfunctioning boxes such as sticking buttonsā¦ Saying things like āyou just have to break the buttons in by pressing them a lot.ā Iām not exactly sure why these people are so quick to give Elektron a free passā¦ These are very expensive instruments. When I pay this much $, I want it to be in perfect conditionā¦ Just my 2 centsā¦
At the time there were no replacement units so it was a choice of put up with it or send it back and wait more weeks/months for a repair/replacement. When I decided to keep mine it was because I had spent the last 2 months anticipating its arrival and was allready onboard with the concept, as a new customer my belief at that time was that elektron made high quality units with good after release development, so when others said the sticky keys were not a big problem and needed ābreaking inā I accepted it along with the other bugs, if the shop I had bought it from had replacement units I may have sent it back but given the situation it seemed like a pita when the replacement might be the same.
Looking back it would have been much better for me musically if I had stayed unaware of the digitakt for an extra year, by that point it should be complete and I could have avoided some of the early adopter sourness. I have decided that I will stay away from threads about new elektron products from now on.
Your situation makes me even more angry with Elektron. Their products are really amazing, just wish they would get their sh!@ togetherā¦ I feel your pain though, itās such a let downā¦ Donāt give up on them yet, they just need time I think. I suggest sending it to them and having them service it. It sucks that you have to wait but it sounds like you donāt have much choiceā¦ Sorry broā¦
Yo mate
i have exactly the same problem on the same button 
i have the digitakt since 2 dayz - let see in the 2 next dayz !!!
My buttons did not loosen up by themself. I fixed the position of the top plate to the buttons. Everything is going smooth now.
I described it in detail here: Sticky buttons - any solution?
Hope it may help someone of you.
Thatās nice of you to provide this guide but, performing an open surgery and thus voiding the warranty isnāt exactly ideal for a new and expensive product.
Be brave and careful. Its no rocket science 
Besides, how will anyone detetect that you opened the device? There are just some simple screws to unscrew and tighten up. No seals to be broken ā¦
This sort of thing needs to stop. Please donāt accept it, send the machine back.
Hey krunchr. This is exactly what I have done. Just a pity I didnt see your solution earlier I had to find out myself after some weeks. I have done a new post so it can be found easier: DIY fix for sticky buttons!.
Your description is just a bit more economic than mine. I needed much more words to say the same. 
Take care
My button 2 had same issue but I fixed it by myself just little bit of pushing the button to opposite direction from the sticking side. I think it needs some know-how to fix it. Buttons seem very sensitively made, and all gaps surrounding the button should be identical. Sometimes when we move the machine, some shocks can be done to the machine and accordingly some buttons start to be sticking, I guess.
Had the Digitakt a couple of weeks and had my first play a couple of days agoā¦ button 6 sticks then pops back up at randomā¦ button 13 catches face plate every timeā¦ Have no such problem with buttons sticking on the Digitone will see how it goes after a few seesions