Projects, action items, lists and key results have different layers of security. If you did not create an action item or project, you may not have been given full access.
The basic access levels are:
Creator: You created the item and you can do anything/everything.
Team: You've been invited to team up or assigned responsibility. In this case, you won't have access to everything, but you can discuss the item and mark it ready for review.
View-only: You've been granted view-only access by the creator. In this case, you'll see a history of updates, but not much else.
More details on security are coming soon.