Hi Nick,
Ah the old screwdriver trick. That’s usually my first port of call too!  Do you try to turn it when it’s powered up? Is there a kickback when it spins up?  I’ve always powered it down first, then give it a few spins with screwdriver and then it seems to go. 
The AC pump on my last boat was exactly the same each spring.
Cheers
Dean
SY Stella
A54-154