Help with D318 Motor/No.6 shovel

I changed the engine oil and filters on a No.6 shovel,( D-6 , D318 motor with a pony motor). When I tried to start it, the engine started and stopped two times. When I tried to start it the third time, the 318 motor appears to be stuck/locked up. Pony motor works fine. I am new to dozers and diesel motors. Is there something other than the obivious that can cause this appearance of being stuck. I am asking this before I start taking the motor apart. Any help or advice would be appreciated. Thanking in advance those who reply.

You have an odd one here but we don,t know the whole story. First don,t start those engines till you have oil pressure up on them esp. after changing the oil & the filters etc. aren,t primed up yet. Make sure the tractor transmission & the clutch is not engaged & the compression released & try to bar (turn) the engine over & see if it,s bound up then. Not much more to tell you to try other than this & get back to us on here & maybe it will shine a little more light on your problem.