We seem to be destined to have this problem throughout our entire life. We had it in the last house, even when we had the bathroom done.
We've moved into a new build and the bath seal is starting to crack and leaves a small gap between the bath and the wall. It must be when the bath is full of water and an adult is sitting in it 
Dh redoes these seals and he does use decent quality materials and he's very good at DIY, so he's not messing them up, yet they continue to break down and end up cracked and messy looking. He's even tried flexible, rubber type sealant, but it still comes away.
I always make sure the area around the bath is well dried after people have bathed or used the shower hose, and I'm not scrubbing it with harsh cleaning materials. Nobody is allowed to shower after he's done it to hopefully give it a good chance to dry.
Can anybody recommend a technique or material that would end up with a better result and preferably last longer than a few weeks?