There may be a couple of reasons for this:

1. There are some activities where guests cannot be assigned to the activity individually.

2. The guest(s) do not meet the age requirement of the activity.

3. If any of the guests' details are missing on the guest details pages within your account.

4. Sometimes, when the booking has been made offline, although the names are fed through and appear to be present in the guest details pages, they do not appear for selection on the activity availability page. In this case please visit the guest details page and click update (you may have to do this for each guest), then the names should be present.