Verm-X isn't a wormer. I may help by making the digestive tract less pleasant for the worms to make their home in, but it won't kill any worms that are already there.
Try flubenvet (this doesn't kill gapeworm however) or an ivermectin spray (you get this from the vet). Ivermectin kills both internal and external parasites and is quite powerful - it also kills gapeworm.
It is really important that you de-parasite your chickens as lice etc. can kill. Don't just think it's red mite that can kill.
Don't rely on the powders either - they aren't powerful enough to kill any lice/mites on the bird. Kitten strength frontline drops will help. Just use the powders to sprinkle into the bedding if you use straw etc.
Feather loss around the rear may be due to Northern Fowl mite - only a permethrin spray will deal with this. Other symptoms include a sort of grey, greasy diarrhoea and sore bum.
Coccidiosis is another relatively common infection which causes hens to stop laying, general loss of condition, sitting around with a hunched up appearance.
Chickens are very adept at hiding the fact that they're ill and often you don't realise there's a problem until it's too late. As a chicken keeper you'll soon develop a keen eye for a sick bird and some detective skills can help you to figure out what's wrong and a lot of things are treatable without going to the vet. Most vets don't know much about chickens anyway.
www.thepoultrygarden.com/phpbb/index.php
This site is very useful and there's some good health information - the people are friendly and helpful too.