The + OPK is strange. It's one of those things where you have to get to the end of the cycle and look back to see if it was working or not, lol. Because if you get AF and you had the + OPK, but you don't have a temp shift, then there is a good chance something is wrong with your therm. But if you get a temp shift in a few days and the + OPK was just a false positive (which does happen) then the therm isn't a problem.
Lol, but all that said, I do REALLY recommend the BD one.