Out there a bit, but...
Postsurgically many patients are not allowed to eat or even drink anything whatsoever in case they have complication requireing going back inside surgically. They want no chance of aspiration, etc. under anesthesia.
Insead of drinking, there are spongelike lollipop things that they are allowed to use to keep their mouth, lips, and tounges moist.
Perhaps using one of these prior to impressing would provide the impression material the envionment and situation more akin to that for which they are formulated, designed and expected to encounter. The impression materials, like anything else, have an operating envelope of expected variables providing increasing and decreasing efficacy within that design range. and this patients oral environment may be just on the outside edge and providing a close, but ultimately ineffiecient, improper or incomplete result.
I have sent a message to a hospital friend and will have the object's name for you Monday if you would like. As it is not a medicant, soley a moisture applicator, it should be available for use by non-MD directed personnel.