Did you test for gapeworm or have a necropsy and verify that it was gapeworm previously?
Have you done the Q-tip test this time? Taking a regular human Q-tip -- having someone catch and hold one of the chickens with the worst symptoms... hold her head up to stretch out her neck a bit.. open her mouth.. hold on to the Q-tip tightly.. and put it down (to the side and to the back) of the throat... reach as far as you can (no, that's not really far with a Q-tip and your fingers, but just a couple of inches, as far as you can).... swab the throat.. swab the throat, then, look at the Q-tip and see what you got.... if there are worms on it, they have gapeworm. If no worms on it, then they don't have gapeworm.
Gapeworm is hard to treat... takes repeated treatments of Valbazen (given to each chicken individually), given orally... Valbazen safely kills ALL types of worms.. but, tapeworms and gapeworm are difficult and take 4 or 5 treatments. Withholding eggs.... no eggs, eating chicken, or hatching eggs for the duration of the treatment.. repeating treatment every ten days, and waiting until the 15th day after the last treatment before eating eggs again.
1/2 CC/ML per standard chicken.. 1/4 CC/ML per bantam... (see posts by dawg53 -- use the search here on BYC for worming with Valbazen)
If you didn't use Valbazen or treat at least 4 times... you may not have gotten rid of the gapeworm, if that's what they had before. (If you started treating verified gapeworm last month, you would probably still be "in treatment" now to effectively treat it... it is difficult and chickens do die before the gapeworm can be effectively treated... but, they can die of other worm infestations as well).
Is she having trouble breathing, wheezing, straining to breathe? Please do the Q-tip test and be a poop watcher to see if there are any other parasites, like tapeworm segments in the poo.... and, even if you don't see worms, they still could have some. Have you checked for mites as well (around the vent, under the wings on the skin)?
I am sorry... good luck. I highly recommend using the Search here on BYC.. and search the symptoms -- lots of information can be found that way.