Yes, One of My Three Striped Floats also, the others, stay submerged.
Weigh the turtle if you can, and Try Superworms SO that it can catch them at the surface.
I have read a possible Lung Probelm (sickness) Although Mine eats fine and as long as it has something to hang onto While it is eating at the surface level it is just fine...This particular Three Strped spends a lot of time on land also.