Most common cause of the symptom
You are working on an Open List. Only the User who created an Item on an Open List is able to update its progress or details.
Insite Teams Projects can have three types of Lists; Open, Internal, and Private. Open Lists are those which are available to all Project Users including Project Admins, Internal Users, and External Users.
It is likely that External Users are not a part of the organisation who owns the Plan. For example, these may be sub-contractors to the organisation or their client or consultants. These External Users can be invited to a Project so that they can view and add to certain Lists (Open Lists) but are blocked from viewing other Lists (Internal Lists).
It is assumed that on Open Lists there are multiple Organisations collaborating together. As such Insite Teams protects the status of items and their details to ensure that different organisations do not have the power to manipulate each other's items for malicious reasons.
There are no solutions as this is not an issue and is a feature of Insite Teams which is in place to protect the organisations who use it. If you need further information on this or you think that this does not suit your usage case then please contact us for more info.
If you are only working on the List with Internal Users then you can update the List privacy settings to Internal where you will be able to update each other's items.