some busy airport is not allow to use for refueling airport.
Something like the top 20 or 30 airports in the game are not allowed to be used as tech stops. LHR, ATL, LAX, etc. In almost all of these cases, there is a nearby smaller airport that can handle tech stops. For example, if your tech stop needs to be near LAX, try ONT.
However, your IAD-GUM route should be going more northerly. Your tech stops need to be in Canada and/or Alaska. Cold Bay, Alaska (CDB) is only 180nm farther away from GUM than HNL is. You didn't mention which game world you are playing in, but if the time period is approaching 1980, be warned that this route will never be profitable with a double tech stop.