Try adding one of the following to your key press section of your macro and use it on boss pulls:
/cast [mod:shift] Battle Potion of Strength
/cast [mod:alt] Battle Potion of Strength
/cast [mod:ctrl] Battle Potion of Strength
Of course, having these potions in your inventory helps. 😉
Lastly, you could be having low numbers because you might have a bad macro (Im not rolling a pally this expac, so I do not know the good ones). Your best bet is a grab a couple of macros and do some timed tests on the dummies and then roll some dungeons with the most promising one.