After deworming if the problem continues, you may want to ask your vet about eosinophilic colitis, which Boxers are predisposed to as a breed. A biopsy would be required for diagnosis. If diagnosed, this can be controlled with various antibiotics and steroids.
Do you have their stools checked often for parasites? Since your dogs live outside and are of course barefoot they are constantly being bombarded by parasites. Parasites have involved life cycles with active and dormant states. The diarrhea may coincide with a parasite infestation. It would be best for your dogs to be dewormed with Drontal Plus periodically, and also have their stools checked for the parasites that Drontal doesn't cover, such as Coccidia and Giardia.