I had been having this problem a lot. At least, I think it might be the same problem you are having. Whenever i would tp or go over a sim crossing there was a chance my body would disappear to others. It would still show up fine to me but others wouldn't see it and I wouldn't know it happened unless someone said something.
After trying to figure it out a while and experimenting I found out that what was happening was that it was detatching to others, but it was still showing as attached to me, so I was seeing it. In my inventory the body would show as attached but I wouldn't be able to detatch it because SL and everyone else saw it as detatched. I would have to relog, after which it would show as detatched, and then attach it again. It got really annoying.
Then, I was randomly talking to someone and she said to try to attach it to the left pec instead of wherever it attaches by default. Since I did that it hasn't happened to me at all. So, I think like someone else was already saying, it probably was conflicting with something else trying to attach to the same spot. That answer really worked for me.