Does Vitex just stop working?!

I had been having irregular periods, so I did some research and discovered vitex! I started using it and that same month I started my period!

The following two months I started my period on exactly the same days and got a positive opk on exactly the same days.

This month, I started testing with my opks right around the time I should ovulate, but never got a positive. I would've kept testing, but I was going on vacation and didn't want to bring them along.

I should've started my period around the 6th/7th of September, but still haven't and have gotten several BFN, so I know that's not the reason.

All in all, has anyone had vitex work for them and then have it stop working? Should I take a break from them and start back up to see if it brings on a period?