Engine Cant figure this one out

Don't know why but my motor cannot be rotated when I install the spark plugs. Without the plugs it rotates fine. Could oil have Gotten into the cylinders ? If I put compressed air into a cylinders and rotate the motor that cylinder holds pressure when both valves are closed and releases air into either the intake or exhaust manifold,as it should.

Hypothetical question. If I removed all the rocker arms so that all the valves are closed and then installed the plugs should I be able to rotate the motor by hand ? The only thing I can think of is the push rod length is too short. I measured using an adjustable push rod checker and if memory serves they were 6.75. This is with aluminum heads 1.6 roller rockers and trick flow stage 1 cam.nothing was decked . Number 1 at tdc on comp stroke and timing marks line up. It doesn't seem to be mechanical. It feels like cylinder pressure is keeping the motor from rotating. None of the spark plug electrodes are crushed or anything.any help greatly appreciated.just really strange that motor rotates by hand with the plugs removed but not when plugs are installed.