I don't think this is necesssarily an unhealthy trait, but it is a puzzling one to most other types. I've noticed that most INFJs process interactions on a delayed schedule and need time to think about anything that's happened between them and another person before they come to a conclusion or resolution. To others this can seem as if they are acting capriciously when they can't connect the current behaviour to the present interaction (it's based on the previous interaction!). It also can make them feel like INFJs are holding onto grudges if they keep bringing up unresolved bits of the same issue. I think for us, it's difficult to really clean out that junk drawer of emotions without someone's help in finding places for everything. If you push an INFJ to resolve something before they've gotten all the pieces put away, it will keep coming up and they will resent you for not taking time to properly understand what the whole issue is before wanting to get it done with.
We also really need to feel truly listened to. If we are listened to without being given extra input while we are still venting frustration or bleeding off extra emotion, we will naturally soon start looking for a practical solution. If not, we will remain stuck on the problem with the added problem of not feeling listened to. You perform an excellent service just by being willing to listen. It's needed for processing, sort of like an emotional equivalent of how House uses his team to sort through his own possible diagnoses.