no surge??

So I have been tracking my temps and using  opks for several months. Each month it seems as though my opks are positive later and later in my cycle. This cycle I am on day 18 and have been tracking since cd 10...still no positive opk. What the hell is going on? I have been testing multiple times a day, 4 hour hold, around the same times from cd 12-18 and nothing...   any insight??? I've had ewcm for several days and some ovulation pain, but not much and my temps are about the same, no real highs or lows. Is it possible I missed it?