Hi,
In one of the machines, did you check the contents of the following file (for Windows) :
C:\ProgramData\OCS Inventory NG\Agent\OCSInventory.log
If the communication between stations and server is not good, you will have the information in this file.
In OCSReports, you can also check one of the following settings :
Configuration > Inventory > Frequency
Regards, Stéphane