mcursua1, has your partner been checked out? It may have nothing to do with you, but with his sperm.
I was on the Depo from Oct 2004-July 2005 (4 shots total) and became pregnant in July 2007 (healthy baby born in April 08 ). Had no type of bleeding what-so-ever for the entire year. My periods became regular again in about Nov-Dec of 05. My periods also became regular 2 months after giving birth, even though I am exclusively breastfeeding (I just think my body likes being regular).
It is possible to still be fertile after the Depo, but I would NOT recommend it to ANYONE, even my worst enemy. If someone is reading this and is considering a type of birth control, there are PLENTY of others that are just as effective. Try the patch, it's a little less convenient than the shot, but more than the pill, and just as effective as both.