There is a 25% chance your item will drop for your class assuming you are the only one on your team in that class.
Now from that you have a 25% or 33% chance of it being a tanking piece.
At worse you only have a 6.25-8.25% of getting a tanking piece.
Odds are already not in your favor. Sin tanks have it the worse as they have 25% while juggs and VGs have a 33% on their class drops.
This is about as accurate an explanation as you are going to get. The only solution is to continue to grind Flashpoints until you get what you want. Sorry, but grinding is part of the MMO experience, and it is not going to be changing any time soon.