Hi, usually DRT only checks whois (that's where expiration date is from) when the domain name does not resolve and availability scanning is on. This ensures that domains that do resolve (which only takes a fraction of a second to check) don't get resources wasted on whois checks (which validate availability).
To turn this off, goto Options->Scan Settings->Whois/Availability tab and UNCHECK "Only check whois availability if primary method failed". Then in Search Engines select "Availability/Parking Status".
That should check whois (and give expiration date) on all domains.
Keep in mind that expiration date and drop date are different. DRT can calculate the drop date (when the domain becomes available again) for .com/.net and other TLDs.
Thanks,
Luc