Confused about OPK. Help?

So I took a opk on cycle day nine and the surge looked fairly strong and so I figured it would get stronger in recent days like last month. Except it hasn't and looks weaker. Last month on CD11 I got a positive surge but today it's not. Should I be worried I missed ovulation? I'm going to keep testing and BD every second day but I'm worried I missed my chance as we didn't start BD until cycle day 9 :(​