I have a vinyl liner IG pool.
While I was out enjoying the pool yesterday I found a spot where the floor felt soft, an area about 4" around or so and perhaps tapered down to 1/4" deep. I could press it down a bit more with my thumb. Then I noticed several other areas with that felt like that. When I closed the pool last night I could see these recessed/soft areas clearly with the light on.
The year we had the pool put in we bought a "Torpedo" toy, you throw it underwater and it shoots along pretty quickly. After a few uses of that thing I saw (also at night with the light on) a lot of little craters pock marking the pool floor where the torpedo had hit. That thing was quickly gotten rid of. I mention this because the floor was soft enough for the torpedo to dent but felt solid under foot. Now in several areas the floor feels kind of soft.
All I can think of is that the ground water has worked its way through the pool base stuff that shapes the floor and has created these soft spots. Seems odd though, we havn't had enough rain to fill a thimble all summer, could the heat we've been having do anything to the pool base? (my water was at 96 degrees for quite a while)