Since these are interference engines, I was referring to correct ignition timing, as in a failing CPS, damp/cracked/bridged dist cap, cross firing wires, malfunctioning CCU, ECM. The kind of thing where you check and see that you have spark, gas etc but it just won't start, because you don't realize it's firing at the wrong time.

On a non-interference engine, like the older Volvos B-23s/B-230s, correct mechanical timing is also essential to start it. If the timing belt jumps or breaks in a non-interference engine it will crank like mad, have spark, gas and everything but it won't start.