What temp. is their cage, basking and cool, and night? Beardies NEED to be very warm, at least 95 F in the basking, and 85 in the cool area. This is very important, and is often neglected in the winter. Bearded Dragons are pigs, and if healthy, will eat until you give them a limit. Them only wanting to eat crickets is because they are junk food junkies, and these are the most palatable to them. Changing up the insect protein is a good idea, but crickets should be the base of the protein portion of their diet. The base for their plant matter diet should be leafy greens such as Dandelion greens, Collard greens, Mustard greens, Kale, Turnip greens, Escarole and Chicory. If they weren't raised eating much plant matter, it may be hard to get them interested in it. Persistence is important because they need fruit and veggie nutrition.
Do you have a UVB light for them? I recommend compact fluorescent or mercury vapor, as their effects reach farther, and last longer( tube fluorescent lamps reach 6 in. and only last 6 months max., Compact fluorescent reaches a foot and lasts a year). This is one of the most vital aspects of beardie care, and it is often neglected. They are very susceptible to MBD, or Calcium deficiency and require high amounts of UVB in order to process calcium. Without much information, my guess would be that the smaller one has developed MBD. Can he hold himself up and support his weight on his legs, or does he shake and stay flat? The sooner you take him to a Reptile Vet, the better chance he has of getting better. Also, make sure you are providing the needed environment and make necessary changes because the other one may develop it as well.