I have been killing hellhounds for hours and no drop, is this normal?

It's completly random, make sure that you dont have any other clues in your bank. It's best to get clues when your least expecting them, and not just hunt for them :P. Best of luck

Check your bank to make sure you don't have any clues in there. And just keep going, it's random. The only thing that will affect a clue drop is whether you have one already.