How to get my Mossberg 940 Pro SPX to cycle cheap ammo?

[MR5quarepants]

Just picked up a Mossberg 940 Pro SPX! It runs 1600 FPS high brass slugs fine, but those are killing me.
I need tips to get this thing to cycle low-brass birdshot reliably so I can just have some fun at the range.

 

[Frank]

Mine loosened up after a few boxes, it just needed time and proper cleaning

 

[True American]

  Mine wasn't a fan of the lighter stuff at first either. But after a bit of a break-in period, it loosened right up and started cycling through more loads. It just needs a bit of time and a good number of rounds put through it.