Hi,
It can happen if these elements were some parts of a label or a dimension that the reservation become corrupted. These kind of elements usually belong to another one (e.g. a door, a wall), so when you reserve these elements you also reserve all its belongings. Another tip would be that maybe there is a "phantom" user in the project - someone who already left the project but the Manager still shows that they are joined. Because of that it can happen that elements which have been reserved by this user cannot be reserved again by another user at the same time.
So first of all I would check if every user is really joined into the project. Then I would ask everyone to leave the project (do not force leave them to make sure you avoid "phantom" users) and save a .PLN version of the actual state of the project (to prevent data loss). After you re-joined you should try to reserve all.
If there is a "phantom" user or you still cannot reserve all the elements, then I would Open & Repair the project then re-share it on the Server.
Regards,
Noemi
Noémi Balogh
Community Expert, Admin