If you are really not ovulating at all, there may be a medical problem. Have you been checked out for PCOS?
If you are just ovulating irregularly, there can be many different reasons. Being overweight or underweight, stressed, sick, or even just getting older can all affect your cycles. I used to have very irregular cycles (BEFORE having kids) - anywhere from 4 weeks to 4 months apart. I was ovulating, just not regularly. When I did ovulate, I had a period 2 weeks later. I was overweight at the time. Since then I've had two kids and lost a lot of weight, and I have much more regular cycles now.
it may be some kind of condition...i would got to a doctor and get it checked out