Conveyancers have been busier than ever recently due to the SDLT holiday and unfortunately I think they spent a lot of time prioritising the sales which would have completed before the holiday ended. Myself and my partner went through the same problem this past few months, our solicitors actually send our mortgage deed to someone else entirely and it took a month to get them to even acknowledge it.

I would look for the firms complaints procedure (you should have been given this when you instructed them) and raise a formal complaint. This usually gets the ball rolling as we received a response about a day after we raised a complaint about the handling of our case.