How can I repair suction cups on grab bars?
No suction cups are 100% impermeable, meaning they always leak SOME amount of air, even if at a molecular level, so they will eventually let go. There is no way to change that, you just need to periodically release them and re-apply them, cleaning the cup and the surface as well when you do. The only viable alternative is to permanently affix the bars, not use suction cups. If you are a renter, that may not be a viable option, so just take the Preventative Maintenance approach.