I've had three AChR-antibody tests (all negative). I was finally diagnosed by a single-fiber EMG (actually, I had three of those, too! The first two were inconclusive).

It's quite common to test negative for the AChR-antibodies at first, and positive later.

There are two other antibody tests you should have: MuSK and LEMS (Lambert Eaton Myasthenic Syndrom). I had one each of those. There are even more antibodies that cause MG-like symptoms, but they're not commonly tested for.

All of this is to say: definitely yes, keep testing!

Abby