We had a new acrylic/fibreglass tub installed with a mortar base but it still feels spongy along sides and part of the middle. I looked underneath and the mortar is only covering the back bottom and a maybe a 6 inch spread in the middle. Is it going to crack eventually? Can this be fixed to be more stable?
If there is movement in the bottom there is a risk of it cracking. Who knows though, it may well last many years. I have heard of using espandable foam insulation under tubs but you need to be careful not to use too much and cause over expansion.