It's not possible to help you with such sparse description of the problem, since boarding vehicles surely do not disappear, and it's not possible this would happen if you installed the latest GSX Full installer, which was a mandatory update the last time we had a major update (Dec. 20th), and this was also indicated in the release notes that you can read when the update notification appears.
So, first be sure you have installed the latest GSX Full installer.
If you are sure you have, then report ALL the following:
- The airplane used and whether you customized it or not.
- The airport used.
- The gate used.