some times its hard to know the systems which are having issues in reporting their inventory and even some systems doesnt reporte anything to the site server though the client is installled on it.
Here is the collection which gives you all computers which doesnt report inventory for x days(taken 15days).
select SMS_R_System.ResourceID,SMS_R_System.ResourceType,SMS_R_System.Name,SMS_R_System.SMSUniqueIdentifier,SMS_R_System.ResourceDomainORWorkgroup,SMS_R_System.Client from SMS_R_System where ResourceId in (select SMS_R_System.ResourceID from SMS_R_System inner join SMS_G_System_WORKSTATION_STATUS on SMS_G_System_WORKSTATION_STATUS.ResourceID = SMS_R_System.ResourceId where DATEDIFF(dd,SMS_G_System_WORKSTATION_STATUS.LastHardwareScan,GetDate()) > 15) or ResourceId not in (select ResourceID from SMS_G_System_WORKSTATION_STATUS)
Good luck in troubleshooting why it doesnt report 
Hints:
solution that i tried got worked.
1.check if you are able to connect to admin$ and see the log file i.e ccmexec.log( service is running or not)
2.check it the machine has problem with WMI repository, this is the most possible issue to resolve. you can the simple batch script to reolve WMI that i posted on my web(http://solodig.com/scripts/how-get-the-rid-of-systems-which-has-wmi-issue/) This can be done Via psexec on list of machines.the script will compile the MOF file with site server and it resolve most of the systems that failed in sending hinv .Use the collection commander tool (http://sourceforge.net/projects/smscollctr/) to intiate the hinv on list of machine single time.it should gives you the result that has been run or not ?
