Rupert the pool butler (aka Smartpool NC52 Wall Climber) would climb the wall in the shallow end, get filled with air and then get stuck at the surface.
There are four holes on Rupert's nose for bleeding air. Drilling out these holes to a larger size solved the problem. Rupert now sinks back down and continues butling around the pool.
Smartpool suggested adding 4 to 8 ounces of weight, but drilling out the holes is much simpler and faster.