Ovulation test

My husband and I have only been trying to conceive a couple months, but last month I took an ovulation test on cycle day 12 in the afternoon and got a positive so I figured I ovulated sometime on cycle day 13. But now this month it is now cycle day 13 and I still am getting negative ovulation tests. Do you always ovulate on the same cycle day or can it change each month? The test like should be darker or the same correct?