There might be various reasons why the staff members aren't able to see the Open Shifts on their portal. Please check the following steps to make sure that the staff members could see the Open Shifts and apply for them.
Make sure that the system default setting Staff Can Apply For Open Shifts is switched ON in the Scheduling Settings. The same setting is also on each site's profile, please make sure that it is switched ON there as well.
2. Position, Certificate and Site match
If you have any of the following settings switched ON in the Scheduling Settings - Require Position Match, Require Certificate Match, Require Preferred Staff Match - you will need to make sure that the staff members have the corresponding positions or certificates attached to them in order for them to be able to see the Open Shifts that are created with those details. With the Preferred Staff Match setting, the staff members have to be have been added to the Preferred List for the site.
If you are using departments you will need to make sure that both the site and the staff members are in the same department. If you have added a site to a department and not added the staff members into it as well then they won't be able to see any open shifts on that site.