You might want to do away with that stuff.
The screen on the 3GS does not work by "pressing". All you have to do is place your finger on whatever key/button you want.
The film is reducing the electrical conductivity of the screen and causing you problems.
Remove the film and you will find that you no long have to press, just touch.
I finally gave up using that stuff on my 2G iPhone. The fingerprints just wipe off.
Take a look in Best Buy, Office Max, Office Depot for alternatives
I currently use the anti-glare Power Support film on my 3GS. I love the anti-fingerprint part, but I don't like the fact that I have to press so hard on the screen with the film on to type, etc..... Is there any other film that doesn't take so much screen functionality away?