Originally Posted by Primate
Look at your bar positioning first before reaching for the bar pad. A lot of the guys I know who had issues like that just had the bar too high/too low or had their hands in the wrong position.
There's also the issue of technique. The foam moves the position of the bar on your back. For example if you're used to doing low bar squats the pad may put the bar in a high position which will affect the lift. Not a huge difference, just something to be aware of.