bearing cap trouble

[TooeleBronco]

I put some new hubs on my trailer. They work great but 2 of them keep loosing the caps. I hammer them in and they fit good and tight but after I drive it 20 miles or so I'm usually missing at least 1 of them. It's never the other 2 but always the same ones. Any suggestions on a cure?

 

[TooeleBronco]

If it matters they are 3500# axles. One is a braking axle and the other an idler. I'm having problems with one hub on both axles. Opposite sides of the trailer too.

 

[DOSS]

I had this and it was due to a cotter pin just barely touching the cap so it would end up spinning it a little and then it would be gone..