Hostname Resolution Health Monitor

Hi

Our SCOM 2012 R2 server some times generates the alert "Cannot resolve Hostname" for HP-UX machines but not all of them. today this alert is generated for a SLES 11 server. 

occurrence of this alert is not in regular periods and every one or two or even 8 hours is happening. DNS configuration is checked out. name resolution is being done from SCOM server and HP-UX server. all the servers are joined a active directory domain. 

what else should I check?

thanks in advance