First thing I'd check if one of my snakes refuses to use a hide is that there's no feces or urates in that hide. Also has the snake outgrown the hide? Could the hide be holding in more heat than you realize and therefore be actually too warm for the snake to want to use it? Some of mine will choose to just curl up behind or on top of their hides some days but then go back to using the hide the next day or later that same day. I'm never sure why it occasionally happens but since it's not every day I don't worry about it much and just figure the snake knows what it wants/needs.
~~Jo~~