The coupons are often usable on solids, or the homespun plaids, which aren't so often on sale. You can rarely use them on the calicos. They're good for the holiday fabrics, IF you can get to them right away before they go on sale! (Sorry, it's already too late for the spring holidays, you had to get to those before Christmas to use the coupons.)
It may be the case that an individual JoAnn's will make an exception to the policy that you can't use the coupon on sale items (using the percent off the regular price)... but it hasn't been that way in my area for several years now.