Why does our sump pump located under our in-ground pool fail over and over again?

When we bought our house with an in-ground pool we had no idea that there was a pump located under the pool in the deep end that pumps out ground water. The people that sold us the house neglected to tell us of its existence and that it not longer worked. After some investigation we determined that they replaced the pump twice and the second pump failed before we took possession of the house. We have subsequently replaced it three times and now it has failed again. Each time we replace the pump, the pool workers come in and pull the liner back, dig out the old one, replace it, wire everything back in again, pull the liner back and fill the pool back up. The first time this cost us over $4,000 because we chose to replace the liner because it was somewhat degraded, the second and third time it cost us about $1600. The pump is wired to a switch above ground on our deck. We flip the switch on when we have a lot of rain to prevent water collecting under the liner causing it to float. Any ideas why each pump would fail?