any suggestions on why i am not ovulating?

Hi. i have between a 26 to 31 day cycle. I have been testing ovulation since the 4th day of my cycle. its day 19 and i still did not have a hormone surge. i use clearblue tests. have you had the same? should i test every 12 hours? i am 42 years old and i have read i should test each 12 hours as these surges are very short for woman above age 35. UPDATE- DAY 24 AND NO OVULATION YET!!!