Invalid OPK??

I have been tracking my OPK for about 4 months, more dedicated this month. (Every day 2 times a day since CD5) 
I noticed a slightly darker OPK on CD10, but no positive OPK.
Today CD13, I used two different brand OPK. The ones I always use showed a very light barely seen negative. But the other brand states no line means it's invalid...all 3 were invalid.
I don't know what I'm doing wrong but I'm getting very discouraged . Any pointers ?
Baby Dust to all👶❤️