Keyless entry problems

I just bought an 02 Blazer. It had 1 key fob with it (FCC ID: Elvatca). It is a 3 button fob - Unlock, Lock & Remote start. Sometimes it will start the car (when the start button is pushed) and some times it wont. It also will NOT lock/unlock the doors. The parking lights will flash which shows me that the car recognises the fob but who cares if the lights flash. I just want the locks to work.

I have tried the "hold unlock while turning on/off twice" routine and the " stand 1.5 meters and hold the lock & unlock on the fob together thing to no avail :banghead: . Any suggestions?

Is it factory OEM or add on.
Make sure battery is good.
Try reprogramming it.