Missing clients in Service Request forms for technicians

Hi.

We have SD+ 8.2.17 and we have synchronized it with our AD.

With my administrator credentials when I create a new service request, I can select the calling client, any calling client, from a list with all clients (users). Some of the technicials, who don't have administrator privileges on the system, can't see ALL the clients. They can see most of our clients, but can't see ALL of them, no matter HOW they search for them (name, login name, dept. etc.).

When I study the parameters of a client who IS visible to technicians, and comparing it with the parameters of a client who is NOT visible to them, there appear to be no differences at all.

I can assign a missing client to a technician request, and then they can see it, but when they try to select a client from the proper screen upon opening a service request, they can't.

How can I solve this? Any ideas? Thank you in advance !