sounds like the heel piece is too close or far away from your boot. Don't crank your dins up, keep them wherever they were set. There is a screw behind the heelpiece that moves it forward and backward. The heel of the boot should cover the whole tab (the piece you actually stomp on to click in). Anywhere between there and another turn forward is ideal spacing.
A relatively low-impact way to check how well they hold (that is, if you don't go to a shop) is to click in, and lean forward like a big nose butter. if you release, you probably need to adjust the spacing or din, but think of spacing first.
Hopefully that helps