Plain answer - you're not generating enough enmity.

You need to do more overpowers and I would recommend cross classing flash from the Paladin to help you manage the TP in instances where you need to generate more aggro than you thought you would.

Also, after 50, you will get a lot of really geared players and if you, yourself are not as geared, there will be times when you have difficulty holding threat on everything.

Start looking at the gear other folks are wearing to see if they are outclassing you, so you can adjust your pull and how many overpowers you need to use to hold the enmity.