It's probably a problem (or problems) with deficient nutrients from lack of fertilizers. I know you weren't thinking it would have taken this long, but they definitely should have been getting more Phosphorus and Potassium over the past three weeks. It's up to you whether or not to give another bloom feeding... I probably wouldn't since you're this close to harvest, and you don't want to leave the buds tasting like chemicals. If your pH is in a good range, you might think about throwing in some blackstrap molasses. About a tablespoon or two per gallon will do just fine. The plants really seem to tolerate and actually enjoy these feedings right up until the chop.