I've been doing a lot of requesting and reading this year. From that has come, sadly, a lot of rejection. When passing, I try to give writers some sense of why the manuscript didn't work for me. Frequently I mention tension, pacing, character development. I think about these emails carefully. I try to mention published books, sometimes movies, as examples of pacing done well, or characters developed in a way that makes them compelling.
This sets up my expectation that when that author queries for a new manuscript (and many of them do) the problems I saw in the first manuscript are solved.
Too often they are not. All too often on a new manuscript from an author I've read before, I get ten or twenty pages into the new book and it's deja vu all over again.
What you should take from this: if you're going to requery agents who've read your work pay special attention to what they said in the pass letter. Chances are they've got a copy of it and they'll remember your previous work.