This is a known scaling problems I have discussed here before after levelling 3 diff warrior tanks you wont be able to get around them till you are 80 and get some gear.
The pally will be stacking strength ignoring all other stats for him strength converts to spell power which means consecrate gets stronger.
Your hit will be abysmal at level 77 I am guessing it will be like lower than 70 and your attack power will be low as well because you will be stuffing stam and defense. Next you probably filled out your prot talent tree but probably haven't got down to deep wounds on the arms tree.
The basic problem translates to alot of thunderclaps miss and you don't have deep wounds so the pally's consecrate quickly overcomes your agro on off targets. As bash said also target the pally and make sure he doesn't have righteous fury which increase his threat by 90%.
Between level 74 and 80 your worst nightmare are ret pallys and elemental shaman dps with lightning bolt spamming.
The easiest way is to simply ask the ret to not use consecrate some will get all anal about it but the scaling at lower levels between classes is not linear nor does blizz go out of there way to make it so.
** Remember Warcraft players fail in directions you never thought possible.