In the Footsteps of a Killer - S6-E7
Plot hole: Spoiler. The killer needs to get rid of two women. One is his mistress that he just dumped and is acting psychotic about it, the other is his wife. He kills the wife and makes it look like it was his mistress to murder her. So she goes to jail, where she will die two years later due to pneumonia...and in all this time, she never once says anything about the affair! He has been extraordinarily lucky, because had she said anything, and she had absolutely no reason not to in her circumstances, the case would have not been so open-and-close, they would have considered the idea that he could have been an accomplice, but even assuming the past detective (which DI Richard Poole called a good detective) was a total fool, at the very least Jack and his team would have found a trace of this controversial alleged affair in the files and solved the case much earlier. But no, the plan was to send his mistress to jail and that somehow made her cease to be a threat to him, when instead she'd have been much more dangerous to him.
In the Footsteps of a Killer - S6-E7
Plot hole: Somehow, Jack is so lax in his investigation that he does not ask any detail about the sales representative supposedly the husband had an affair with and that he ended (which could have very well been a suspect nobody considered before, since he ended the relationship to stay with his wife...had she existed, but Jack can't know that he's lying!), but has acquired DNA from Ian Matlock to run an overnight test on the hair sample (which we have to assume was complete with follicles and still in test shape after 8 years in a bag).
In the Footsteps of a Killer - S6-E7
Plot hole: The original murder went unnoticed because of extraordinary incompetence of the police; even if they did not have the ingenuity to extrapolate the background noise, they still had to investigate how a murderer would have gotten away with a corpse in the middle of a carnival, but the issue is never raised. More importantly, the last phone call of the victim came from a place 40 minutes away from the city, which would mean a different cell tower - and since they needed to track down the body that never turned out, monitoring the phone the last call came from is standard procedure.
Answer: There's probably no particular reason. Sets and props on long-running TV shows often change as needed and for various reasons throughout a series run.
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