I suspect the problem was that one of your first couple of stays did not credit properly to the promotion, either it was not compliant in some way with the terms and conditions, or just failed to be captured by the system; the brands should be interchangeable in this type of promotion. One word of advice is that for the previous Hilton promotion my AmEx credit did not appear for a few weeks, unlike how it usually credits a day or two after the final charges have posted.