Will not sturt but turns over

Hello
I AM having a issue with a 2005 Chevolet monte carlo it has the 3800 motor.
I have spark, I have 50 psi of fuel pressure and cannot figure this problem out when you crank the motor over it kind of acts like it is hard to crank but trys to start. I have replace the ccrankshaft position sensor the ignition the camshaft position sensor the map sensor and the maf sensor Also I have checked all the fuses reset the security light and replace battery and also the coil packs the ignition module have all been replced and still will not start so now I am confused what do you think it could be?