Frustrating opk's help!

I've been using opk's religiously this fertile window, I have only ever had faint lines show up in the past which I put down to not testing enough during the window and missing the surge. Anyway the past 2 days I've tested to get faint lines on my opk which were very slightly getting darker I last tested at 7 pm last night before going to bed hoping that this morning I would finally get a positive O l, I tested this morning and NOTHING not even a sniff of a faint line just undeniably negative! What the hell! Did I surge overnight while sleeping or have I just not ovulated? I'm so frustrated with the whole ttc, I have yet to have one positive on anything! I can't even get a positive ovulation test. Please any help or advice would be greatly appreciated