moopdog's been through this conversation before, but just as a refresher:
II uses a combination of several features to determine whether a request can be satisfied with available inventory. The two most important are often referred to as "trade power" and "resort quality". There are a few other restrictions, but these two are the big ones.
Trade power expresses the expected supply of weeks at a particular resort/in a particular area/at a particular time in II's inventory compared to the expected demand for those weeks. It is important to recognize that this doesn't necessarily track general cash rental trends---it is peculiar to II's inventory. II wants to reward owners who turn in weeks with high demand/low supply, so as to encourage those owners to deposit more time. So, the other weeks in inventory that are in very high demand are not given to the first owner who requests them---rather the first owner willing to give up a similarly popular week. In trade power, more is always better---you can always "trade down", but can't always "trade up".
The other major component of the equal exchange concept is resort quality. This is primarily determined by comment cards turned in by exchange guests, but there are other factors. Again, II wants to reward guests who turn in luxurious units, so guests with lower-quality resorts are not able to exchange into the plushest weeks available, even if they ask first. However, II also doesn't want to deal with a lot of unhappy members, so owners who deposit very high-quality weeks are not allowed to exchange into lower-quality resorts, because those resorts are likely to leave the owner feeling short-changed and wanting to complain. So, resort quality can restrict trades in both directions---you can't get into a resort that is much much nicer than the one you own, but you also can't get into a resort that is much much worse.
DVC resorts get very very high exchange evaluations. So, most unsuccessful exchange requests into DVC seem to be limited by the upward quality filter, not the trade power filter----especially in low-demand times for Florida generally, as September certainly is. DVC weeks have been available for exchange almost continuously in late August and September for the past several months.
During Flexchange, which is 59 days or less before the check-in date, all trade power restrictions are waived, as are some quality restrictions. As I understand it, the upward quality restriction is significantly relaxed, but the downward quality restriction still applies in most cases. So, many people who can't get a DVC match in advance often find them available during Flexchange.
Edited to add: others have said the same thing more simply than I could have. I also agree with utahkennedys: Horizons is none too shabby a place.
