This is a glitch of some sort. I used to think the same thing, but I knew you ran into the same issue when I noticed u were RDM.

The only common occurrence between every time surecast fails to give you the buff of surecast, but goes on CD, is when dual cast is up.
Once i was on WHM, i never had this issue pop up, and realized i could surecast all the bosses i couldnt as RDM.