Check the antenna cable, they're located on the right side of the phone. If I can remember they are a blue cable which connects the antenna from the bottom to the logic board via that antenna cable. Also check the bottom antenna housing to see if it is aligned properly.
To check this properly, you will need to take out the two gold phillips head screws then push the antenna casing in a direction towards a 45° angle not too hard of course then while doing this, screw the phillips head screws back in. This should make sure that the antenna pins under the housing have full contact with the antenna covers.