possible problems:
Con-rod is wrong length.
glow plug is fouling the cylinder head. Test by removing plug and trying to turn over.
Piston gudgeon pin is fouling the transfer ports; cylinder liner is not aligned correctly, gudgeon pin is not secured properly.
Con rod is not symmetrical. It has a front and a back. The front has a rounded machining of the big end bearing web to clear the crank pin support. The back side does not have it and fouls the support preventing the crank from rotating fully if assembled the wrong way round.
Piston ring upside down and is fouling the lip in the top of the cylinder liner that comes with wear.
Debris in the intake venturi fouling the crankshaft intake slot.
Debris in the bearings.
A fold in the space-time continuum that manifests as an inability to rotate Super Tigre cranks. (OK maybe not, just been watching Stargate SG-1)