maybe they didnt put the spline back in the prop on reassembly -easy miss.
Do you mean the key-way turning the prop?
If it were mine, I'd check the right prop first. If it is correct, my next step would pull the prop and double check there is a key way there. If those checked out then I'd double check you RPMs. Do you have access to a timing light with a digital readout? If you do put it on plug wire number 1 and check the actual RPMs while on the water. If that checks out, its your Tach. Tachs do go bad, and it could be as simple as a loose connection.