Is it a digging dog? I only ask because if the dog isn't being used for digging then it could be down to what its eating.
I had the problem with my terriers eyes going gooey, after feeding them minced red meat from a certain supplier, I took him to the vets and they gave me eye drops that cost me over £20, which didn't work. Once I stopped feeding him the minced red meat, his eyes were sound, nothing wrong with them at all.
How have you tried to clean them?