Old Dog, New Tricks
You can teach "old dogs new tricks," but it takes more time and patience. By old, I mean over a year. I know that is not old, but there is a change in maturity around that age.
Best kept secret with a sheltie, is day one home from the breeder, you should be teaching the bark and the no bark command.
Day One: work on house breaking, recall and barking on command. I have read that many of you use sound association training and that is the way to go with this. We use 4 to 6 keys hanging from our belt for the sound, but if you use a clicker that works to.
Teach your puppy to bark when you say "speak" and then mark the silence with the sound (keys, click) and say "No speak."
Then you can teach what is acceptable to bark at and what is not. You have got to start very early 10 weeks, before the pup finds his own voice (around 16 or 20 weeks).
Now, Jam has been with us since 10 weeks............... and Jam has this down pat. He sees a raccoon, "Jam Speak." and he does........He sees someone in the driveway and we are out in the fields, he speaks and comes and finds.
Crackers, good golly molly, the dog is as smart as a whip (if a little homely for a sheltie, he never came out of his teenage awkwardness phase), but he is definitely trying our patience. He barks at every little thing, the smell of deer, a branch out of the tree, a bird in the morning, everything. In the past two weeks, I have said "no speak" 5 million times. Jam even looks exasperated with him. The other day, before I could get "no speak" out and Crackers quieted down, Jam body slammed him sideways, as much to say, "will you please learn the
no speak command, you are driving us all nuts."
Teach your shelties to "no speak" from day one at home because we have never had this problem with a barking sheltie until this little guy, Crackers, came to live with us at 13 months old, and had no training for the first year.
I just keep telling myself, you can teach old dogs new tricks! (David is still trainable, he's an old dog

)