What Causes Irregular Periods In Spite Of Being On Contraceptive Pills?

I'm 23 and i have a bicornuate uterus. I have been taking the 3 months Depo contraceptive for a year and i had stop taking it for a year now i started taking it again for the same 3 months i am seeing my period for a long while and the doctor recommend me to take Femaine contraceptive with it for me to stop seeing my period, i had for a month and it stop but within a few extra days am seeing my period again, what can be the cause for that?