How to get PRTG to update DNS lookups

We noticed a number of IP addresses that weren't being resolved, so corrected those in DNS. We also cleared the DNS cache on the PRTG server and checked that it would now resolve the IP to a name, but PRTG continues to report only the IP address. How do you force it to reread the DNS? (I understand you wouldn't want to do this continuously as a large list of IPs would slow down the whole system if it had to reresolve at regular intervals - but you should be able to force it when required.