Especially if they are a small breed, and the person who applies the ring is not very careful, then the testes themselves can pop back through the ring, so only empty 'purse' is removed.
Yes the vet can deal with the problem, either by open castration or using the Burdizzo, but get a quote first. Is it impossible to leave them as they are? We leave many of ours entire anyway (to see by 16 months which are possibles for breeding), and they get on OK together, apart from a little jockeying for position.