I had the same problem when I first started riding. I'm not a small girl on the bottom - and because of this, I was buying my bike shorts in Mediums and Larges (depending on the brand). I kept getting the chamois all bunched up and in places that it didn't belong.

When I was trying to fine tune my saddle as my rides got longer, I finally measured my sit bones. Lo and behold, I had discovered my problem! My sit bones are quite narrow for a woman. And they do NOT match the width of my rear or my legs (which are both large). My anatomy and my body shape don't align very well and I never knew it until I started cyclilng.

Now I buy most of my shorts in smalls so that the chamois fits me and therefore doesn't move. What a world of difference! You need to buy your shorts to fit your hips and crotch, not your rear, your stomach or your legs.

My guess is that your shorts are too big.