Proximity Sensor not working after screen replacement?

I've repaired hundreds of iphone 4 screens, and for AT&T 4's I always cut a hole in a small piece of electrical tape and place it in front of the proximity sensor when replacing a screen. This method always works for me, but with verizon iphone 4, there is already a foam insert around the prox sensor. The foam seems to do the same thing as the electrical tape I would insert into an AT&T iphone 4. Despite this foam insert, I've noticed that every once in a while the prox sensor won't work after installation of a new screen. iPhone 4s has a similar foam insert, but I've never had a problem with iphone 4S prox sensors. Does anyone know why the verizon phones are so faulty? Should I just rip the foam insert out of the phone and simply stick some electrical tape with a hole around the prox sensor like with the AT&T phones? Any insight would be great.