What could be causing me to have periods even though i am having monthly injections of decapeptyl 3mg?

Decapeptyl is a gonadotropin releasing hormone analog, but after initial stimulation of hormone production by stimulation of the pituitary, continued use will inhibit the production of hormones including estrogen and progesterone, which will generally stop ovulation/menstruation. Monthly injections at "3 mg." may not be sufficient to produce the desired effect.