What is the chances of a antibiotic for chlamydia not working ?

Chlamydia: If the medication is taken properly it should work. Often the improper intake of the medication and the lack of treatment of the partner or re-exposure to the infection through unprotected sex is the reason for the failure of the treatment.

Zero: Chlamydia is very well treated with either Doxycycline or azithromycin.

Zero to 15%: The two main treatments are doxycycline 100 mg twice daily for 7 days or azithromycin 1 gram single dose. Doxy is100% effective if taken as directed, with no missed doses. Evolving research shows azithro is about 95% effective, i.e. it fails in 1 in 20 patients, up to 10-15% if the rectum is infected. All patients should be retested after ~3 mo to detect both treatment failure and reinfection.