Shared mailbox giving issues

I’ve got a problem with a shared 365 mailbox where it would tell users the message store has reached it’s maximum size. There are about 10 users working on the mailbox. I manually added the mailbox after turning of Auto Mapping via Powershell. Not one user has an OST larger than 15GB, and the error specifically mentions the shared mailbox. The mailbox is only using about 3GB currently. Any ideas? Thanks!

Edit: Cached Exchange Mode is off, Mailbox is only online.