Contractor Dispute — CPA made error on tax return

I've been trying to resolve this on my own but I'm stuck.

CPA made error on tax return. I've been dealing with this for about 5 months now and the situation isn't improving.

I have already done some research online but did not get a clear answer.

Do I have a strong case? What should my next steps be?

I've dealt with this before.

In my case, it took about 1-3 months to resolve. The key was escalating to a supervisor/manager.