First you must determine if the floor is sagging or the tub is flexing.
I gotta tell you my friend (and don't take this personally) there are some real cheesy tubs on the market these days being sold at home centers. You get what you pay for in acrylic tubs and even then the quality control is sometimes lacking in some areas. I have seen top-o-the-line Kohler tubs with the top flange so far out of wack they should be ashamed it ever went to market.
The cheaper tubs (not Kohler) flex considerably under the stresses of water and occupant weight.
. This causes the lips/flanges to move as you describe.