Jeep cherokee starting problem

HELP! Ok, the other day i went to start my jeep after dinner and it would not start. It is turning over but not starting. We replaced the starter relay, the starter, the pick coil, the distributor, the plugs and plug wires, and i am at a loss on what is the problem. We checked the timing and it is on and it is getting fuel,so what it up? if you can help please do, i am at my whitts end! :banghead: