Been awhile since my last visit.
You may have a parasite problem but it is also possible that the smaller discus never had the potential to grow normally . If they were purchased from a fish shop i would be especially be inclined to suspect they endured less than optimal care which left them partially stunted. It takes a surprisingly short time in a fish shop where they may have only been fed flakes twice a day for a week or two to permanently stunt them.
If you bought them all at the same time, same sizes, and from a specialist, then the lack of growth may be more related to the position they have in the pecking order. If these are your first discus then you may have been unintentionally responsible in failing to observe early signs of their falling behind in time. Not hard to do if you aren't experienced.
There is a possibility your Discus have a fluke problem. I do recommend the meds suggested so far. PraziPro is gentle on the fish and tough on the flukes. Try it since it does no harm and do follow the directions. It may be also added to their favorite pellets. Just saturate the pellets prior to using them first. It doesn't hurt to treat from within and without. Assuming you Discus are aquarium raised and not wilds, they can contract fluke from wild Plecos. Both need the same procedures to clean them up.
A 55 gal tank is not a very good discus aquariums. You get a very poor surface area to volume ratio. Even a 40 gal breeder has more surface area than a 55.
I recommend using a 75 gal at the least as a permanent home for 6 to 8 discus. You are staying within the rule of thumb of 10 gal/discus and as long as you are diligently changing 50% of the water 2 X week, then you can keep 5 adult discus Ok in your 55 gal.
I was given a 55 gal and have used it as a temporary grow out tank for 1 inch fry and then to when I sell them at 2-1/4 to 2-1/2 inches. I change 50% of the water daily, have both an Eheim Classic #2217, large sponge filter driven by a MaxiJet 600 power head and a wet/dry filter running 500 gph. This allows me to grow out about 2 dozen discus to salable size.
Try to provide your water test results and filter details if your discus are still in trouble..