Its possible that you might have O'd between cd 71 and 73 because you had EWCM and your temps started rising to a higher level than all of your previous ones. I have learned the hard way that WHENEVER you see fertile CM do the BD even if FF has told you you have O'd already because it is just going by your past temps and cant predict what the future ones will be after O - it might just assume that you have low post O temps when in fact once you O your temps go a lot higher than your pre O temps. Does that make sense? Dont place too much trust in FF, just keep BD every few days throughout your entire cycle, and possibly every day that you see fertile CM.

Hope that helps!