Shower base movement

I installed a new fiberglass shower in our second floor master bath. I installed the base as per instructions by placing 6 or 8 blobs of thinset morter and pushing the base down on the morter. Everything was fine for about 6 months then I noticed the base would move a little when you stepped on it and got a little worse over the next few months. It has been over a year since I installed it and I still have movement. I am afraid this will eventually cause the development cracks in the base. I may be able to access it thru the back wall or making a hole in the cealing under it by the drain. Does anyone have any suggestions how to fix this problem?

I've heard of filling voids under fiberglass tubs and shower bases by going from underneath and filling the void w/ expanding foam. Some say the best answer is to rip it out and do it again.

BTW: Not 20 minutes ago I just did the same thing for our new fiberglass tub. Looks like the mortar wasn't quite thick enough in one spot. The rest of the tub is solid besides the one area I was feeling movement. I was able to drill from underneath. First through the subfloor. Then, w/ a masonry bit I went through the mortar w/ a depth stop so I didn't go too far. (Since I knew approx how thick the mortar was.) Popped through the mortar. Shot foam in and filled it up. Curing now, we'll see how it goes.

If your mix of thin set mortar was too wet, it may have spread too easily and sunk more than you want it to. in cases that I have done, I try to use a cement sandmix with a dryer consitancy

What kind of foam did you use?

Great Stuff. I used the large gap/crack filler (Black can) because it's what I had.

If you try foam, you want to use the low-expansion stuff that stays flexible, otherwise, you'll crush the stuff into a powder after awhile.

Thinset is not designed to be used in a layer greater than about 1/4" or so. If it is too thick, it can take awhile to cure, so if you tread on the base too soon, you'd mush it out of the way, and then have no support. Deck mud, or a sand mix gets pretty dense fairly quickly, but neither should really be walked on for 12-24 hours.