What Causes Periods Continuously After Birth Control Injections?

Hello, I am a 21 year old female and currently taking the birth control shot. I have been on it for a year and have been having a continuous period since I have started the shots. The dr I go to said it was normal and by the 2nd or 3rd shot he said my body would get used to it and the periods would decrease or go away all together. Why am I still on my period after being on birth control for a year?