It should not be if done correctly - do you see a spinning earth/globe icon?
If so, go into internet recovery's disk utility and format individual partitions first (not the entire physical volume). I format them all to Mac journaled and when you've erased all partitions, then you should be able to select the physical drive and run another erase.
the last time i tried messing with a bootcamp partition it left 50gb of unused space in a grey area and when i tried to reclaim it said error could not unmount disk. I had to enter internet recovery and reformat my whole hd as 1 partition and redo everything