3G iPad - No Sim Card Installed

Hi

I have a iPad 2 GSM 3G

I replaced the digitizer on it, I am unsure if it related by it does not recognise any sim cards.

I check the continuity with the multimeter from the sim tray contact points and no wires/traces are broken.

For some reason when I remove the screen for the second time it magically got recognised again. It worked for a few hours and overnight it has stopped working again.

Is the screen possibly blocking the antenna or shorting it out or something?

Thanks