With the spark plugs removed, it should roll smoothly through every revolution. If the plugs are still IN, then you're feeling the cylinders coming up on their compression strokes and that's normal.
Try what PackPack suggests. Pull the plugs and check to see that it rolls smoothly. Then install Starter and try to Start it. Starter should engage and genset should rotate smoothly. Then roll it again by hand. There shouldn't be any resistance from the Starter. You'll know the starter's releasing and there's no interference.
It could be the genset happened to be shut down with a piston exactly at Dead Center. Where it stops going Up and hasn't started going Down. If the genset than sat that way a long time, the piston could have gotten sticky in that position. They're much harder to crank than when piston happened is part way.
