When some users create a project the project isn't being shared as defined in their access level.
When a user creates a project Workfront will first use the user's "Set as my project access template" and then the user's assigned access level settings.
Have the user go to a project > Project Actions > Sharing > click the gear icon in the upper right. Next to the Set as my project access template click the half-circle arrow icon. This will Reset to Defaults. Now the next time the user creates a new project Workfront will read the user's access level and follow it's settings.
To verify the access level settings go to Setup > Access Levels > click on the user's access level > choose Projects > Edit button > under Create, you may have "When someone creates a project, grant..." for example: Their Company -> Access to ->> Contribute.