I bought some visa gift cards at giftcards.com which were advertised on bigcrumbs as a 2.4% rebate and I was initially credited only $2 per card a couple of days before bigcrumbs published that change in their offer. The missing credit "wizard" would not allow me to submit an inquiry until two weeks had elapsed since the purchase (if I recall correctly). By that time, bigcrumbs had posted an additional credit to my account to cover the difference exactly. So, as far as I can tell, all my credits eventually posted.

Obviously, if the credit is still missing once enough time has elapsed so that bigcrumbs will allow you to submit an inquiry about it, you should do so.

Update 4/15/2011: A credit for the difference posted to my account by the time I was eligible to submit a complaint with their "wizard." So, now everything seems to be OK between bigcrumbs and me.