IF they are 500-600 heals above you, you will naturally be behind them in heals, unless they are really bad heals. Get better gear and you will catch up, you are probablt not doing much wrong.
Also know that wild growth and CoH are wildly overpowered to what you can do as far as raid heals at the moment, and they are spammable instant casts. By the time you cast your chain heal, the whole raid is already healed. Not much you can do about that.
Be happy that you are keeping your raid up and progressing, get better gear and you should catch up.