Can you provide me with the versions of the 3CX Mobile App and 3CX Windows App that you're running? Are you sure that you're logged in as the correct user on the Windows App? You can check this via Settings >> Configure Accounts on the Windows App.
Are you sending a message from the creator of the group to the participants when the group is made on the Android App? If you don't send a message, the group doesn't show up until you do.