I have had no experience with OPK's or even Maybe Baby, but from what i do know about it, id say its likely you did ovulate yesterday, and your hormones werent at their peak?? What time did you do the OPK test? Ive heard the best time to test is around 2pm, when it has been found that the hormone peaks, or something like that.
If it were me id take it as O day, even though the second line wasnt as dark or darker as the control line.