Can non mobile qbs not slide sometimes?

If you are sprinting they will dive.. You need to release the turbo button before sliding.....

Really? Before it's always been simply tap the X button... Now it makes him dive

I've started sliding earlier with Brees, just to be safe.  Seems like he puts the ball on the ground when defenders make eye contact

Both triggers + X on xb1, should work every time unless you do it to late.