If your cycle is 28 days, then your most likely to ov around day 14, so I would test say day 10 on wards. Most people have a luthal phase of 14 days, so if you have a 31 day cycle, then you probably ov on day 17 so test from say day 13 onwards. Hopefully then you should catch the surge. Good luck though, it sounds so simple but isnt really. They are quite hard to read because the lines have to be darker than the control line. Clearblue digitals are the best but quite expensive.