pH could be entirely relevant. You're asking for help, why not answer a simple question?
The pad should be in the drip tray not on top of the balls - if it's on top of the balls, dirt can easily cause the water to follow only a few specific paths off of the pad and not spread correctly on the balls at all. Especially in the case of the type of pad you're using. I strongly suspect removing that white batting and putting some proper flat blue/white aquarium prefilter in the bottom of the drip tray will solve your problem.
I don't understand what you mean putting the pad in the drip tray... I don't think you know what type of wet dry filter I have.
