If you have bought the lcd with frame, it's the frame. The original one has some contacts which transfers the signal through the body to the bottom. If it doesn't have these contacts on the new frame, you'll get very low signal(actually you will only have signal on open spaces with a nearby antenna)
If not, there are 2 culprits: One is the antenna cable, and the other is the small chip that the antenna cable connects to the bottom of the phone.