I've heard stories of Detectives ignoring someone's pleas to check some evidence that would prove their innocence. What incentive would a detective have in NOT

I've heard stories of Detectives ignoring someone's pleas to check some evidence that would prove their innocence. What incentive would a detective have in NOT checking out the person's claim? Do they just assume the person is lying and that their intel on this person is solid? In a case like this, it seem plausible from the beginning that the CI could be lying, (at least as the story is told) perhaps there are just no consequences for these detectives when getting it wrong. It just seems like it's worth checking this guy's Surveillance Camera Footage sooner rather than later.