Sweeps that don't folow the rules
Open the rules. At the top, or the very bottom you should find the name of the Sponsor and the name of the Promoter (AKA: Administrator).
The Sponsor puts up the prize and the Promoter runs the sweep. Normally, problems like yours are solved by the Promoter and that's who I'd write to.
Most sweeps in the "traditional" sweeps categories have both a Sponsor and a Promoter. Occasionally, you'll see a sweep where the Sponsor and Promoter are one and the same. Usually, those are run by large companies with experience in running sweeps.
There are various State and Federal laws to be complied with and most large companies want to comply. Occasionally, you'll see sweeps that exclude certain States. This is probably due to State laws regarding some sweeps. California's liquor law is one example.
It's possible that to save costs, some of the "newer" sweeps run by blogs, pinterest, twitter etc. might try to skip the use of a Promoter, and not be conversant with the State and Federal laws that impact their sweep.
The Federal Trade Commission -
FTC Bureau of Consumer Protection - is the Regulatory Agency and you can file an online complaint on their website.
I suspect the FTC is more interested in large promotional scams rather than the small stuff.
A complaint filed with the FTC with a copy to the Sponsor and Promoter of the sweep might have an attention grabbing effect and get you some action from the sweep.
Not to be negative, but Is this a $5 or a $500 item? Is it really worth your time and trouble to pursue it?