I see your truck is a few years old. Is there a possibility that you have air getting into the tank? I do know that DEF will crystallize if exposed to air for long periods. Does your truck sit for long periods with the DEF tank low?
I'm just trying to think of other possibilities because it seem you would have noticed a foreign substance around the filler. Heck, I can barely pour the DEF in with a spout on the can without spilling a little. It would be hard to put a substance like sugar or something in that little hole without spilling some.