Question about Site-Targeted Ads

I'm using Google site-targeted ad placements to promote a few of my products.

I've noticed that over the last few days that I am not getting any impressions on these couple of sites.

How do I find out why? These sites have been cash cows for me in the past.

I assume its either because the site owner has blocked my ads or competitors are outbidding me. Does Google tell you if a certain site has blocked you?

Could be the holidays fall off that hits almost all businesses this time of year.
Could be you're being out bid.
Probably not, privacy issues so you don't go get the offending site DDOSed or some such silly thing.