If the primary is good, then you need to dig deeper. The above mentioned video probe would be good to see if something is in the cylinder. My next thought would be to open tappet door, and the points cover to see if anything is jammed there. Remove the valve covers and see if a valve has been sucked into chamber. Barring problems there, the timing cover would be next, perhaps a broken tooth on the oil pump spindle, broken tooth on timing gears, pinion gear, distributor shaft etc. Try to consider it a puzzle that will take a while to complete. Make it fun, half the fun of and RE is finding the problem, then fixing it.