> a severe testicle infection,could this be the reason they missed
Well infection can certainly affect fertility. One of my dogs had an infection in his scrotum, a course of antibiotics and it cleared up. 2 months later he had a collection to send semen to NZ and all the little swimmers were dead or deformed. It was 6 months and several collections later before his semen was viable again. The vet thought it was the heat of the infection that caused the problem.
As to whether the infection could have been brewing 3 months before it really surfaced I don't know?