Building equal muscle mass

I am working hard on increasing muscle and losing fat.  My question is...  my right arm is significantly stronger and more toned that my left.  I have been working for months to try to even this out but it seems the gap has only increased.  I already exercise them separtately, Im never lifting the same weight with both arms.  Any suggestions or techniques?

You should be lifting the same weight with both arms.  Choose a weight that challenges your left arm and do those reps first.  Then repeat with your right arm.  It won't be as challenging, but it'll help even out the imbalance and, eventually, you'll get the left one strong enough that they're both challenged by the same weight.

^She said it all. 

When I work with people whose strength comes primarily from daily life functions (as opposed to the weight-room) these kind of imbalances are very common.

Regarding corrective training, try to lift the amount of weight that challenges the weaker arm but do so in in unilateral movements where the stronger side can't compensate.  This is where dumbbells are far superior to barbells (and fixed- motion machines, ofcourse). Working on compound lifts (not just isolation arm exercises) may also balance your strength faster. 

 At my gym, I've recently started using the Freemotion lat pulldown which allows for unilateral movement - and I'm feeling the results much more than with a traditional bar-style lat pulldown.