Does it spit out oil or fuel when cranking with plug removed? If it does the cause of fluid getting to combustion chamber has to corrected.Fluids do not compress and you get a hydrualic lock.

If this isn't the case, you have a ACR or automatic compression release problem. This can be either a failed ACR on the cam requiring the replacement of the cam or a mis-adjusted valve clearance with a too wide gap. The too wide clearance gap negates the action of the ACR which only opens the X valve around .005" during a portion of the compression stroke.
LMRM; Bob