OPK struggles

Hi ladies. My husband and I have been ttc since May, and the last couple months I’ve been using opk just to try and better pin point my ovulation day. The results seem to be all over the place and I’m finding it hard to judge, except for the day I did one twice I’ve been doing it the same time roughly in the morning. Is it normal to show a positive and then later that evening show a negative, When the day before was negative? Suggestions would be great!