Ovulate different days each month?

Hi, so I think this might be a dumb question, but I don't know and can't find a clear answer anywhere.

I just started using an opk last month, so this is my second time trying. Last month I clearly peaked on day 12, but today is day 12 and the opk shows a clear negative. Is it normal to have a different ovulation day each month (or LH surge, rather, since that's what it's testing for)? I have endometriosis and have been TTC for 17 months, so I'm on the lookout for any issues.

I am going to keep testing and will hopefully get that positive opk, but I'm just curious what you ladies have experienced and if you've gotten a positive opk on different days of your cycle.

Thanks!