It's not a terrible thing, I wouldn't stress that bad.
Do you have enough spots for them to hide or perch up higher. It maybe that they have tried to sit up higher and your plants aren't supporting them of they feel a bit exposed if you don't have sufficient coverage up the top.
If you have lots of places closer to the ground where it is more denser they will tend to stay down there because they feel more secure.
Also are we talking about day or night? My WTF never leave the floor at night hardly, Climbing all around, sitting on the water bowl, at the side, hunting under things for any left over crix etc.
Daytime they are basically up the top but I have made the top of the tank very dense so they feel hidden and secure while they sleep.
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