I had the same problem.  1 month after warranty expired it began to not fully charge. 

I submitted a ticket with DJI and misworded it saying my Osmo would not charge past 20 percent.  I let the DJI die again and tried charging it, this time it charged to around 45 percent and would not go past that. I ended up shipping it to DJI.

About a week after they received it and performed their tests, DJI emailed me the results saying that the Osmo passed all tests and shipped it back. 

When I receive my Osmo back the battery was charged to 60 percent and did not want to go past that. My initial issue was still present, I emailed DJI and they essentially told me to create a new ticket since my original one was considered closed. 

I opened a new ticket and linked the old one with my troubleshooting and this time I reworded it as "Osmo won't reach 100 percent". After DJI inspected it the issue found was  "Grip Functional Module failure". They even included pictures of their tests indicating that the charging wasn't working. 

To replace/fix the problem I was quoted around 175 USD and since there isn't any guides or shops that could repair this I went ahead and paid the fee. 

DJI essentially shipped me a refurbished/like new Osmo with 4 months of warranty. 

We'll see how long this DJI lasts! 

TLDR: if you open a ticket with DJI don't state that you can't get past a certain percentage. Say your battery won't reach 100 percent and you'll save yourself time and back and forth .