1. Check that your seat is straight and that the nose is not too high or too low.  All of those things will affect how your bibs react to your seated position. Also, too far forward or rearward  will adversely affect the fit.  Perforated saddles can help, too.  Lastly, check your saddle height as that can affect it, too. 

2. When trying bibs, be sure that the legs stay put...if they are too loose on your leg, they will immediately creep up your leg affecting how the chamois moves.  It will not stay in place.  I have this issue in a large size, (I am 6'2") as I have thin legs and on many top brands, the  lycra just will not stay put.  I prefer the grippers to the flimsy material strip many put on the legs these days these days.

I have had great luck with Sportful...made for professional cyclists.  They seem to stay put on my legs, and thus the very excellent chamois also stays put.  

3. I used to have the issue with sitting squarely on my saddle...for some reason I sat more on one side than the other, correcting that through thoughtful riding fixed that issue.  Have a good riding buddy take a look from behind while you are riding. 

Hope these ideas can help. 

Aaro Paavo Heinonen