I presume the clients are showing as connected to the console - they have an orange tray icon applet showing "managed by XXXX" if you hover the mouse over it, rather than an icon with a red X showing "looking for XXX" if you hover the mouse over it.
If the clients are showing connected at their end but not showing in the console, and they are Dells, what model of Dell? Dell GX270s have a manufacturing issue which results in many GX270's having identical BIOS IDs and even serial numbers, and this causes the GSS2 console to misidentify them.
We do have a development fix for this, which you can get from technical support (or by emailing me at nigel dot bree at gmail dot com).