Tight Gain Knobs

I opened my DJM 900-NXS to clean it up inside then when I put everything back together the gain knobs became really tight. I noticed they go back to normal if I unscrew the screws on the back of the mixer. I removed the plastic brushes and put them back on to see if it would make any difference but apparently it's the screws that are making my gain knobs tight. Hope someone can help me. Thanks!