There could be a couple of reasons:
Your boots are too big or your boots are not correctly done up.

This will depend on the manufacturer, but essentially they should be drawn in tight, so that foot movement within the boot is restricted. The boot should then flex around the ankle. The boot should feel snug even before you do the buckles up, but be sure not to deform the foot within the boot otherwise after 15-20 minutes your foot will start to complain. It should be a fairly relaxed, but snug fit.

At Filarinskis we have dealt with many skiers, over many years and we are proud to offer our expert guidance on the matter. For more detailed advice please consult Filarinskis' trained staff.