I know you I knew you

I've noticed a trend that I'm not particularly fond of. The idea has been mulling about my brain for a while, but I've only just nailed it down. The longer I haven't seen an acquintance(usually female, in this case), the more shy i am in approaching them. Why this is, I don't know.
Here's the weird part of this. I'm perfecly ok when I email them. It's only when I'm with them one-on-one that I clam up.
The first was D. We shared horror stories about job interviewing and developed little writing exercises so that we wouldn't go crazy. I think we got on pretty well. However, time goes on, I got another job, as did she, and I never really saw her again. The one time I did was during an inventory evening when she came in to help out. It had been so long since I had seen her, and I only really approached her once. I was at the registers for the whole evening and she was at the information counter. I went up there once, akwardly said "hello," and walked back up to register because I "thought" I had a customer. I saw her once in a town on the deleware, but I couldn't get past the shyness, so I kind of walked the other way.
Overall, I was thankful that she was around at the bookstore, and I hope I was supportive as she was too me.
The next one is more confusing to me. I really like this girl, and I would have realyl dug being her friend. However, I work part time, and she has left the bookstore altogether. I email her frequently, and she often responds, so we still have a fairly strong acquintance.
I've seen her twice at the bookstore, and both times, I've tried to pretend I didn't see her. This shyness is annoying and breaks me up, and I don't know where it comes from. She just came back from a trip to Italy, so it's not like there would be nothing for us to talk about.
This happened again just tonight. I went in researching a car (I'm a geek), and she had come in to meet her good friend. I passed her as she was reading a magazine. I clammed up again, and I couldn't say anything. I don't even know if she saw me.
It might be better if she didn't, because I wouldn't want her to get the wrong impression. I do like her, even though the actions I take don't quite match up.
I will need to work on that. I can't clam up when I see people whom I haven't seen in a while, but I really want to catch up with.
I feel I've missed another opportunity, and who knows if I get another.