Replying to Heel piece sliding off

i recently got new boots that were the proper size and i adjusted my bindings (marker squires). today for some reason i re-checked how the boots fit into the bindings and i was surprised to note that my right heel piece had slid back into its original setting. i started to re-tighten it but at the slightest pressure the heel piece slid forward clean off the track or the plate or whatever it's called. i gave my other heel piece a tug to for comparison and it also slid clean off. what is going on?