There's a lot of factors that contribute to how employees show up in some places and not other in DigiQuatics, sometimes resulting in "missing" employees. No need for panic, here's a couple places to check the settings before contacting support if they really seem to be completely gone.
Did you double check that their profile is active? Employees can get deactivated after the end of a season or even if you accidentally updated your employee in bulk using the spreadsheet. Check the Active / Inactive toggle on the Employees page to check both areas for employees. You might also want to change your location filter from your primary location to All Locations to see if perhaps they are assigned to a location that is different than your primary.
You can hide employees from showing up on the schedule completely, say a Pool Manager, for example. Check out their profile and make sure "Visible on Schedule" is set to "Yes" under the Admin section.
In employee mode, only employees with the location you are viewing set as a "primary" location on their profile will show up. For example, if I have Blackrock Community Center as a primary on my profile and Green Mountain Rec Center as non-primary, when the employee mode of the schedule is viewed, I will show up in the list on Blackrock Community Center only.
Need to grab an employee from another location? Just change the location of the shift to the other location, select the employee from the updated employee list, then change the shift's location back to what it was before.
In general, DigiQuatics is designed to work for organizations with one location all the way up to potentially hundreds. It's configured so that people can manage small groups of employees, or view everyone if they need to. Therefore, you can only see employees that have a mutual location with you. If you only have access to Blackrock Community Center and Green Mountain Rec Center and another employee only has access to the APEX Center, you simply won't see them anywhere in DigiQuatics. Your super admin will need to give either one of you access to a mutual location so you can view their information.