Honestly, I think it's fine to just take your pick of reasons why you are not happy and convey that. You don't need to be an expert or have a master crib-sheet. Your honest feelings on the issue as they are real, organic, and coming straight from an individual -some of the more important attributes to have for a message being sent to a representative imho.

I've always suspected that form letters and chain emails get mentally binned fairly easily quickly, as some of the comments by other users in this thread seem to reinforce.