I've had problems at only two agencies with my stuff: IS and DT. I've found that only a few reviewers reject images for being too similar, and if I feel strongly enough about an image I'll wait a few weeks and then resubmit it. I don't need to do this very often, though, because I make 4 versions of each shot (horizontal, vertical, square, panoramic - two with copyspace, two without) so that if one is rejected I have three to take its place.
For reasons that I will not get into, I no longer specialize in aircraft shots at DT. But that's okay - my aircraft shots dramatically underperform there anyway (56% of my DT portfolio is aircraft, but only 26% of my sales come from it). C'est la vie.
As far as a slow month goes, the problems at IS and increased competition are having an impact - sales have been below $100 for the past week.