95 lhs starter problem

I've recently completed an engine swap. The problem I have is with the starter. Sometimes it works, somtimes it doesn't. I can hear the solenoid engage but it won't turn the engine. The battery is new, and connections all look good. I had the starter checked recently, and it checked out ok. I've hit it with a hammer which worked twice, but no more. Could the starter have failed since the test? or could the test not have detected the problem?