OPKs don't become positive until just before you ovulate, when your body releases the hormone that causes the follicle to rupture and release the egg. Before you actually ovulate, it's all just preparation anyway (you can get a positive OPK and still not ovulate). So you might not be close enough to ovulation for the OPK to pick it up yet.
Or, you could have missed your LH surge (the hormone OPKs detect). Sometimes some women's LH surge isn't high enough at the right time to register a positive OPK.
Those would be the most likely explanations, in my opinion.
at 10:38 PM on Jan. 10, 2011