2003 Grand Am GT will crank but wont start
 

I have a 2003 Pontiac grand am v6 3.4L. I last week I tried to start the car early in the morning and it would just crank and crank and crank but would not fire up. A few hours later I gave it one last shot before I had it towed and it fired right up. okay no big deal, the next day the same issue occurred once i had it towed the shop tried it and it started right up. they let it sit until the next day and it again did not start. they replaced the crank sensor along with a modual under or around the spark plugs (i understand that it controls the spark) I'm not too good with cars so cut me some slack. Anyways they replaced those parts said it was all fine but this morning i tried again and still nothing!!! Any ideas. My boss tells me he thinks its a security issue but the security light is only on when all the other dash lights are on prior to starting the car, once i turn the key the security light goes off. at this point I will take any advice I can get. Please Help me!!