I am a Christian. Don't worry, I'm less of an evangelist and more of an apologist. (But don't get me wrong, an apologist doesn't offer apologies. They make arguments. The two words just happen to look alike.) I just wanted to say that as a way to explain this post. There are a lot of agnostics and atheists that I respect and admire, and the thing that keeps them in those camps is a lack of evidence for the existence of God. There aren't any verifiable miracles.
A believers' life is filled with tiny anecdotes, small miracles that affirm their faith, but these are useless as proof to the unbeliever. Most of them don't matter. My faith was forged on prayers of "help me get through this part of the video game." You can't get sillier than that for answered prayer, but that was seriously the foundation I started on.
My Little Miracle
My biggest and more recent little miracle was actually substantial and was seen as answered prayers by my family as well as myself. In the hospital, I had some of the best surgeons in the state. The man who worked on my shoulder is particularly skilled, and he didn't think he'd be able to save it. The plan was, I think, to pack whatever was left of the shoulder bulb back into the socket and see what it would do. The other option, which he thought I'd get stuck with, was a total shoulder replacement. You get three or four of those in a lifetime, and they last less than a decade each: I'd lose the use of my right arm before retirement age. When I got into surgery, he opened the shoulder up and discovered that the pieces were bigger than expected. They'd hold screws. He was able to reconstruct my shoulder and I will regain almost full use of it. A small miracle. (My wife would like to point out that me being alive and not paralyzed is also something of a miracle. My vertebra broke in such a way as to not threaten my spinal cord at all. If it'd broken differently, I might have no use of my legs. While those are good abstract miracles, and I'm grateful for them, they don't illustrate my point as well.)
Small miracle it may be, that's just an anecdote. It's no evidence. There's plenty of explanations outside of "Man in the Sky put together bigger pieces when the radiologist wasn't looking." What the well-meaning and open-minded agnostic wants and needs is a big miracle. Make the sun's shadow stand still on the stairs sort of miracle. I don't believe that will happen for a couple reasons.
A Dearth of Proof -- Why Abscence of Evidence is Necessary
Theologically, this is about faith and love. Love must be freely given to mean anything. Now imagine that the God of Christianity is real and an active, obvious interventionist. He's always doing some obvious miracle: ending starvation or curing cancer. Doesn't that compel or at least obligate adoration? But such "love" isn't freely given, and is thus worthless and meaningless. Not even our parents or kids get love that's not freely given -- there are plenty of folks who don't love their parents or don't live their children, as hard as that might be for some of us to imagine. To love a good, omnipresent, omnipotent being demands that being choose not to exert their will obviously. God cannot work big miracles without jeopardizing the thing He wants: our love and praise, freely and thankfully given.
Personally, I also believe that God created a universe based on laws and rules -- physics, biology, science in all its forms -- and populated it with beings of curiosity and intellect -- us, humans -- because it gives Him joy and pride as we figure the universe out. In ignorant times, He has more freedom to bend and break His own rules that govern the universe because doing so would not undermine the knowledge and understanding His beloved humans had developed of His creation. In the modern era, a big, physics-defying miracle would not earn belief. It would just hurt our understanding of the universe and confuse us. That doesn't seem like something that God would want.
Let's Not Fight
I guess I'm saying that there something of an impasse. An individual may experience something which they cannot explain outside of a miracle, but that experience will never be enough to convince everyone of miracles. I'll just have to be happy and grateful for the miracles in my life and not worry if anyone else believes them or finds them significant.