There are several reasons why this kind of thing happens. 1. Is the floor stable or does it deflect. When fitting a bath its always a good idea to use some type of bearers under the legs (4x1 25mmx100mm)2. Has the bath got a bonded baseboard,( By this I mean the fibreglass matting completely wraps around the baseboard and is part of the bath casting) The cheaper type baths, the board is just stuck on and to be honest not very good quality. 3.The leg sections are another problem with the cheaper baths being W section, fitting into plastic tubes, held together with self tapping screws, (utter rubbish) and will deflect like hell. The best type are T section chanells with good quality adjusting bolts, washers, and nuts you can lock on the steel T section. Also the 5th leg is very important to keep the centre of the base board rigid. These should ALL be screwed into the load spreading bearers. Finally prior to tilling seal the bath with dow corning easing it away from from the wall to inject the silicon. Some people fit wall restraints but to be honest most are so flimsy there not worth the effort, especially into plasterboard. the double seal of silicone will take care of this. Follow this guide and you will never get a problem with fitting a bath again. Like I have,nt in 30years. I also meant to say to use 2x1 battens to support the bath frame screwed to a floor batten and the bath timber rim, these can also be used to fix any bath panels. Regards chrisey