If you can no longer book meetings, check to see if there are still meetings slots available. To do this, click on the My Visit button (the name of the button may change depending on the organizer) and go to the Meetings tab.
If you have a lot of meetings that have been pending for several days, don't hesitate to cancel some of them to free meetings slots. Pending meetings block your meetings slots.
If you are unable to book meetings with certain people, the organizer may have chosen stricter meeting rules.