Water Behind Vinyl Liner

I have an in ground pool with a vinyl liner. It is about 9 years old. Last summer, I started to get water under the liner. Specifically, a very large bubble formed in the shallow end of the pool. I put a hose behind the liner and pumped all of the water out. I continued to do this every couple of days until we closed the pool for the winter. When I removed the cover to open the pool this summer, there was no water behind the liner. However, after opening up the pool and getting everything running, the bubble in the shallow end started to form again. I had a pool company come out and take a look and they thought that it could be a drainage issue (ground water). But that doesn't sounds right to me. It seems to me that if it was a drainage problem 1) I wouldn't experienced this long before now (nothing has changed in the last 9 years) and 2) there would've been water behind the liner when I opened up the pool. Any thoughts?