Training in Insight does not make you a human lie detector. Too often in D&D a PC rolls Insight and on a successful check knows immediately if a person is lying. This shouldn’t always be the case. Sometimes you need to work at it. If you’ve never met this person before how do you know that they’re lying. Everybody lies to some extent. Figuring out if the lie they just told you impacts your current line of questioning or not requires work.
For situations where more than one simple roll is required, a structured skill challenge may be more suitable. This is not to say that every attempt to detect a falsehood requires anything this complex, but it might be a good idea to remind the PCs that sifting through an intricate web of lies often takes time and many skill checks.