Blocked ID's for Private Plane Histories

[RWJ OnesGV]

I'm a college student, and I'm doing some research looking at private plane travel. I've noticed, as I watch the "real time" tracking sites, that some I-D's are "blocked." What does this mean?

Anyway, I've still been somewhat lucky by using other sites to come up with pages and pages of flight plan archives for some "blocked" planes, and very little for others. Anybody know why this is?

Thanks a bunch! 🙂