why will my engine crank but not start?

i recently had to change front head (left) because of left exhaust camshaft damage because of bad oil seal. replaced both head gaskets, timing belt, front exhaust camshaft etc. triple checked timing of cam to crank and reassembled. engine turns over but won't start. Confirming that i do have spark and using starting fluid for fuel to rule those issues, why will it not start if spark and fuel do exist? does not even partially want to fire.