Well both Davis and Scott is right, however, here in good old South Africa, we know all about post traumatic stress disorder, that I assure you I have several guys off, and back and off again, including myself. This is a very violent country to work in as a police officer, and especially K9.
My first dog was hit over the head with a brick, during riots here, I was stabbed in the same incident, and many officers got shot or hurt, and since then he was changed, and I guess so where we.
We took sick leave after the incident, I took him out from active service, and we just fooled around, with him having a droopy look, I thought he was out of it for good. Until he was back and declared fit for service, we worked him on man work and built him up again. He grew in confidence, but was like a flat battery, I got I new dog to work with, but kept taking him out on duty then one day he just snapped back.
We had riots here (again) and hauled in all our dogs, and true to form, they were throwing bricks at us. He tackled several, but one person, who was picking up a brick and swing it at him, went down with 34 stitches to the leg with one bite, he bit into the upper leg, and turned his head, and got out, this just snapped the upper leg muscle.
So yes, some come back harder and some just never seem to make it back at all just to much, and on second thought, dont think it is bad selection, every living thing has a threshold to continued violence - maybe we gave up on them to soon. ???