This tends to happen with overly-dried watercolor cakes, especially if you hold the pan upside-down. One way to secure the pan in its place is to get a Blu-tack, or a White tack, tear off a tiny piece, and stick it to the back of your watercolor cake.
You can also do the same thing with a little bit of gum Arabic. Drop a bit of gum Arabic on the pan and then put the dried cake back into the slot. It will turn out perfectly fine!