Clear Blue Advanced Digital OPK Help!!

Hi ladies! 
​This is my second month using the CB advanced digital OPK. Last month I began testing on CD10. I received flashing smileys from CD13-16 and then a solid smiley on CD17. This month I was away until CD10 so didn't begin testing until CD11. I'm now on CD15 and still solid circles (negative)! Do you think I Od early and missed it or Oing late maybe? I assumed it would be around the same time every month so now I'm worried I'm not ovulating? I'd hate to have a whole month "wasted"! Any insight or experience would be greatly appreciated!