i once had a cat with the same problem so i went 2 da vet 2 ask 4 help. he gave me a brochure which had like some steps to help stop the hair loss...
if i remember fine there was only 1 step...
1. make sure the collar is not 2 tight- 2 fingers should b able 2 fit in da space & make sure da skin is not red or flaky.
it is a very common prob. and hair will grow back in 6 mnths. lol