Tanks, don't beat yourself up.
Flash is not meant to hold hate off people attacking different things. How simple would our jobs be if it was just Flash Flash, FB, RB, Flash, Flash, FB, RB?
Flash has two purposes--locking everything on you in the pull and keeping everything off the healer. You and the healer should be the only ones creating any significant hate off targets you aren't attacking.
On boss fights, this changes sometimes but you are not doing something wrong if someone is attacking the wrong target.
Marking is part of my Shield Lob macro. I pick the most dangerous (usually largest) mob in the pack, lob that, flash when i'm close enough, flash again for a boost to enmity and then I do a single target rotation, and then flash again.
On large pulls, it might seem like I flash too much, but you cannot depend on being missed in a large pull, the immunity doesn't matter, your healer will keep you alive if you dodge red, try to keep things in front of your shield, and employ sleeps if your group has them.
As your target is almost dead, pick a new target, I've found that dps will switch to my marked target after the last is dead if I already have it selected.
I get lots of compliments on my tanking and then someone usually starts talking about their last tank, which I ignore.
Flash is capable of everything you need it to be.
Try to be nice. People acting like morons don't deserve your kindness but rude only begets rudeness. Just say "Please attack my marked target. It's easier on the healer if one person is being attacked and things go down as fast as possible--and the only way to do that is for us all to attack the same thing.
Tabbing around when you're comfortable on hate, to check enmity levels, not to wake everything, is a good thing, but I feel horrible for these tanks talking about needing to switch targets constantly.