From the
MSDvet manual :
"The gapeworm Syngamus trachea inhabits the trachea and lungs of many domestic and various wild birds. Infection may occur directly by ingestion of infective eggs or larvae; however, severe field infection is associated with ingestion of transport hosts such as earthworms, snails, slugs, and arthropods (eg, flies).
Many gapeworm larvae may encyst and survive within a single invertebrate for years. Although gapeworms are not a problem in confinement-reared poultry, they cause serious economic losses in game-farm pens and in range-reared chickens, pheasants, turkeys, and peacocks.
Cyathostoma bronchialis is the gapeworm that infects geese and ducks.
Fenbendazole is approved for chickens and turkeys in the USA against
Ascaridia galli and
Heterakis gallinarum when administered in the drinking water at a dosage of 1 mg/kg for 5 consecutive days. It is has also been shown to be effective against
Ascaris spp. when administered once at 10–50 mg/kg; if needed the treatment can be repeated after 10 days. At 10–50 mg/kg, fenbendazole when administered daily over 5 days is effective against
Capillaria.
Fenbendazole is also effective against other nematodes when administered at 10–50 mg/kg/day for 3–5 days or as a single dosage of 20–100 mg/kg, or when added to the drinking water at 125 mg/L for 5 days or to the feed at 100 mg/kg. At 20 mg/kg for 3–4 days, it effectively removes gapeworms in pheasants. Toxicity has been reported in pigeons that received fenbendazole at the rate of 30 mg/kg for 5 days. Fenbendazole should not be administered during molt, because it may interfere with feather regrowth.
(Source: https://www.msdvetmanual.com/poultry/helminthiasis/helminthiasis-in-poultry#v3341937)
You could try again using a different dewormer this time:
"
Tetramisole at 40 mg/kg,
flubendazole at 30 ppm in feed, and
ivermectin 1% at 10 mg/mL in water were effective in removing
A galli,
H gallinarum, and
Capillaria spp in chickens.
Tetramisole at 3.6 mg/kg for 3 consecutive days in the drinking water removes gapeworms. Poultry treated while larvae are migrating in the body develop immunity to gapeworms, even though therapy may abort larval migration."
Flubendazole and ivermectin will treat gapeworms as well.
