Not having a slashing debuff really hurts your dps and the drks dps by quite a bit what i'd propose doing is:

-Getting your Drk or you to swap to War
-Recruit a Nin not only for slashing but for threat up and threat down
-Being out of Shield Oath
-Communicate with your healers to see where the issue is

You being in Shield Oath 100% of the time with no slashing debuff you're going to stay in the 800-900 range until some changes are made or healers are more comfortable with you being in sword oath.