Owners at flaoting week Marriott resorts are generally not permitted to reserve/deposit a week with II until 12 months ahead of check-in. With II, most people seem to wait until considerably later than that. So it's really not surprising if you have been unable to confirm an exchange this soon.
FWIW, those are very, very popular resorts within Marriott's internal exchange system. They rarely match to a non-Marriott owner's exchange request. If it's possible though, you ought to get it, if you can be patient enough. You're using a strong trader and you're being very flexible with your request. The biggest problem that some folks have with this kind of trade, is that you may match it with only a little time to search out a decent price on airfare.
If you don't match something by mid-autumn, you may want to expand your list of acceptable resorts, as Diane described. It depends on whether you would be happy with any other selection, in light of the possibility that you may not get anything until very last minute otherwise. Hope it works out for you.