Am currently on CD 16. Normally I get at least a readable OPK or Persona stick on CD 15 or 16, with usually a line of maybe 3/4 the darkness of the control line on the previous day.
I had some of that nice gooey stuff from about CD9-13 (but I usually get it ridiculously early, and then in patches) and a line of maybe 1/2 control darkness on about CD12? 13? Then nothing. White as white. CM back to something not very stringy. Boobage slightly sore but no screams of agony as I take off the bra.
It's possible that my innards have gone on strike after being messed with last month but now I don't know when or whether to start the pessaries.