If they were large roundworms, yes, pyrantel should have treated them, but if your vet is not used to looking at chicken fecals it's possible that they were cecal worms, not large roundworms. I'll try to find som pictures so you can see what I mean.
Safeguard (fenbendazole) liquid for goats/cattle will treat large roundworms and cecal worms with a single oral dose of 50mg/kg, which works out to about 0.23ml per pound. Repeat in ten days. Gapeworms need 3 consecutive days and capillary worms need 5 consecutive days, but probably no need to worry about them if your vet is sure about what he/she is seeing. Capillary worms look almost exactly like whip worms.
Valbazen (albendazole) liquid will treat almost all worms with a single oral dose of 20mg/kg, which works out to about 0.08ml per pound. Repeat in ten days. Gapes would require 3 days, but I think the one day dose does treat capillary worms.
Wazine (piperazine) will treat large roundworms, but that's it. I think the dose is 1 ounce per gallon of water for 24 hours. I have not used Wazine, so do read up on it.
Tractor Supply sells Wazine and Safeguard. Wazine is in the poultry section, Safeguard is in the cattle/goat section.
Jeffers sells 500ml bottles of Valbazen for about $45 plus shipping, but I think shipping is free on orders over $60.
There is a link in my signature for a 60ml bottle of Valbazen for $15 plus $11 shipping.
-Kathy