Lack of Results
I find it extremely frustrating when you're working with a client who is really motivated and is committed to getting in shape, that just can't reach their goals, no matter how much effort they put into them.
Now I'm not talking about people that just want to define the muscles and great bodies they already have, I'm talking about people who seriously need/want to get into shape and lose some weight to better their health. It's frustrating on both ends.
One, it is hard to see someone struggling so much with this kind of thing. Two, you feel like you haven't done your job to the best of your abilities if you can't help them reach their goals.