Is the surface completely steel? Are there any moulding or gaps they are trying to bridge?
It sounds like wind isn't an issue so rounding corners probably doesn't matter all that much.
is the surface clean they are applying to? In general, are they applying the magnet correctly? Is it going on flat with 100% contact with a magnetic receptive surface?
Every time a customer told me their 30mil magnetic was falling off, I went to the site or looked at their trucks and found they were putting them on wrong.
The 20mil doesn't have the same holding power, but in your circumstances it might work better. They also make a 15 mil.
BTW: This might not apply here, but I always replaced the missing magnets free of charge one time as I saw it as my responsibility to educate my customers, and if I didn't do that when I handed them the magnets, then that was my own fault.