One thing that can make a huge difference is UVB lighting! You need a 10.0
UVB bulb that your dragon can get close to. I hang mine right in the cage. Since the bulbs do not get hot it is not a danger to them as long as I don't mist the electrical parts.
Another reason may be the other dragon, sounds like he may be more dominate. I almost lost one like that before, she was so submissive to the other she just stopped everything, gave up! I had to spoon feed her baby food, and seperate them, etc. Now she is so spunky all the time!
If you gave more details on the set up: size, temp, foods (specifics), etc it would help more, but the lighting and dominance/subbmission have been two huge factors I have worked with.
One last thing that my beardies love, and really has perked them up. I moved them out of tanks into wire rabbit/ferret cages! I know it is not the norm, but it amazing how much they love it! Since they are semi arboreal it gives them the room to climb. They run right up the side, just like climbing a ladder! The cage has removable platforms, so you can always redecorate! They can choose if they want to be 3 inches away from a basking/UVB spot or 2 feet away!
So good luck!