Swol is our healer person so he can give you a more in-depth response probably Monday.
Speaking generally and not being able to see your specific case, have you made sure that all appropriate raid buffs/consumables are enabled in the options? Sometimes I see users with those turned off and it has a big impact.
You can also create a snapshot of your exact character and setup for us to check out, instructions on how to do that here (also let us know if you are looking at Best in Bags or Best in Slot):