Ahh ok, are you using cheapies or an expensive brand? It is possible to miss your surge if only testing once a day, even if it's the same time every day. When you see them get really dark and know you're close, if possible it's better to test multiple times. I've had it be negative in the am, positive in the afternoon, negative at night and negative again by the next day... so if I was only testing say in the morning, I would have missed it
How long are your cycles normally?
I took a peak at your chart and those are definitely pre-O temps, but with that huge dip if your cycles are around 30 days it looks like you should O soon!