Couldn't it just be as simple as she did less than a full season of TNG, while the others did seven years and four movies together? Not too surprising if she doesn't have as strong of a bond with them.
For most of the TNG cast, it was 15 years of their life. For Denise Crosby, it was a job she did for less than a year and then a few guest spots thereafter.
No, I don't buy it. She was a main character, had a love interest with anotehr main character, re-priased her character twice that I know of after her death, and then had a daughter who created a new story line (which was never resolved; also, her sister was even the focus of an entire episode). She was in the series finale for goodness sakes. She voiced herself character at least one Trek game, was in both Trekkies films, and even played a relative Yar in a fan film. Plus she's been in other projects (convention fans don't always ask Trek actors abotu Trek -- there's plenty to ask her about). I've seen lesser bit part guests stars get more attention at conventions via youtube videos. She looks a little uncomfortable and quite frankly, as I remember in one, like she's just walked off the street having been living there for a year. The cast seemed to interact nicely with each other in various conventions, but both times I saw her, she didn't.

Okay, fine. Everyone in the TNG cast hates her because she ran over Patrick Stewart's dog. Happy now?
I totally just made that up, but since you insist on seeing a secret hidden answer where there likely
isn't one...
Maybe she's just uncomfortable doing conventions because she doesn't like public speaking or being the focus of attention as herself. A lot of actors are surprisingly shy and introverted.
For crying out loud, I left a job
last year and I don't really talk to anyone from there any more. A few I'm Facebook friends with, but that's it. There's not some big secret reason. Sometimes people just fall out of touch.
They were coworkers in the late 80s, not people who took a blood oath to remain lifelong friends.