WOW! 6 children?! Congratulations! That's amazing! My husband and I would love to be able to have just 1! (long story) Anyway, from what I've heard it can take a couple of years of being off the depo shot before being able to conceive, since it's been over 2 years since you've been off of it, I reccomend getting an appointment with your gynecologist and seeing what type of treatment can be done to help with the process of becoming pregnant.
If you stop getting Depo-Provera injections, your period usually returns within 3 to 10 months but sometimes it may take 12-18 mths..I suggest that you consult your gynecologist and get yourself examined to rule out other causes of not conceiving as it has been close to 2 years since you have stopped the depo shot and fertility should return by this time.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.