Have you protected the filters or powerheads. I have a bta that got stuck in my overflow when it was knew, post is on here somewhere. Sometimes they will hide away when they are new to acclimate to the light.
What are your tank perameters? It could be unhappy with your water
Sometimes they will put their foot under a rock and then stretch out to get the light. This can take them a few days
If they are really unhappy/stressed/Ill they can just melt away but this would normally make your tank cloudy.
If you are really worried you can hunt for it just be careful moving rock incase it's foot is attached between 2 rocks.