I took my dog out in the garden this week and he had a big stone in his mouth I asked him to drop it but he just sat there looking at me I took charlie in doors and he still wouldnt drop the stone, he was holding it very tight, now tonight I said bed charlie and on his way to the crate he picked up a chew and same happened again for 15 mins he sat with the chew in his mouth I tried to take it out his mouth and he started to growl very lightly, he eventually put it on the carpet so I picked it up, I am frustrated as to why this is happening,What am I doing wrong, he wont do as I say.