You need to have it in bright light, they're normally a full sun plant. Are you giving it any fertiliser? And what temperatures are you keeping it in? These are all pretty important issues for it.

room temps are between 70-80 F.

I used to have another mango seedling right next to it that sprouted leaves after several months. I have never given it any fertilizer. Would diluted urine work?

Those temps are okay for night time but on the cool side for day time. So that during the day with low light could be what's causing the weak growth.

Urine is something which would be better put through a composting process. It's highly variable in its contents and wouldn't give you the balance that a plant requires. You could use organic seaweed or fish emulsion fertilisers, they would be more likely to give you a better balance.