Floor under liner crumbling

I had my liner replace a few years ago. Prior to the liner being replaced the floor and slant were smooth. Now there are missing and crumbling parts of the floor which cause the liner to sink a few inches in several places. It is also bumpy and not smooth. When they pulled the cover off it was without a liner for about a month before they finally installed the new one. I am not sure what the surface on the bottom of the pool is made of but it looked like it was some sort of plaster. Anyone have any ideas what could be causing this and more importantly how I can fix it? I assume I will use this liner until it needs replacing and then deal with the problem. I am starting year 3 on new liner.