Keeping Stalag 13 escape-proof

It must have been a major problem - for the writers - to keep up the record of no successful escapes from Stalag 13 in the TV show Hogan's Heroes. After all, one of their main missions was to expedite escapes, and thus in the program there was a veritable avalanche of prisoners passing into and out of the camp - and yet no one ever escaped, supposedly. How did the writers come up with enough plausible explanations for that fictitious record so that Colonel Klink could maintain his claim? Also, what about the other regular prisoners in the camp? Hogan and his staff were there on special assignment and could not leave, but surely that was not also true of all the other guys who were there. Was it a case of "once here, always here?" Obviously, this was comedy and, like the shipwrecked souls on Gilligan's Island, fun was more important than consistency.