No method of contraception is 100% always use multiple amounts, condoms and depo...

Are you wanting to get pregnant but have taken the depo injection? In which case, odds are low that you would become pregnant. You would have to wait until the injection wears off and even after that it can take a bit of time for some women.

If you have become pregnant after having the injection, then as adviceishere said, it happens to a small percentage of women since no birth control is going to be effective 100% of the time. Or it could also be due to missing the window of when the next injection was due to be given and getting it late.