2002 Chevy Suburban cranks, won't start

Hey everybody, I just registered and am new to this site. I'm getting frustrated with my suburban. It was running fine, no problems whatsoever. I went out to start it and the starter was engaging the flywheel, but not turning over. I replaced the starter and it started up, but rough at first. I drove it around, came back, shut it off, started it again then shut it off. Thinking that i had fixed it, I left it alone. Now its back to where it was. I took the new starter back and it was bad already, so i put another one in and it wont start this time. Cranks and sounds like its trying to turn over, but just wont. Any ideas/ suggestions PLEASE? I dont know what to do here. I ran a diagnostic and no trip 1 dtcs.

What engine and sub size?
If engine is turning over good you need to check for lose of spark to plugs or lose of fuel pressure while the engine is cranking.
If you are losing spark that is your problem.
If your fuel pressure is not up to specs that is your problem.
Post back on spark and fuel pressure readings.

Also any security light staying on and after market anti theft struff and remote start stuff?