why not ovulate?

My last normal period was Jan.11 but this month I had it early and it was more like spotting than a period. I took a pregnancy test before calculating my ovulation and it was negative. We started watching ovulation the day after my period went off and it has been about 3wks straight we been tracking still no LH surge. In about a week or so my next period will be here. Why am I not ovulating? I have had previous children with no problem so I should be fertile.