Your dog must always feel it's safe to come to you
Where possible in the early stages of training, the dog's return to you should be made worthwhile, even exciting. Use a pleasant tone and a little fuss to show the dog you're pleased that he's returned.
If the dog doesn't respond to this, the chances are that it's finding being somewhere else more interesting than being with you!
You can resort to the (strictly temporary) use of titbits to encourage the dog back (although we never need to use titbits for training) and whenever possible grade the reward according to the response you get - most fuss (or titbit) when the dog obeys instantly - polite acknowledgement if the response is reluctant.
If you need to shout at your dog after it's done something wrong, think carefully about it. It's best to correct a dog at the precise time of the offence (or preferably just before) rather than later - but sometimes you have no choice.
When training your dog with sheep, don't call the dog back to you and then shout at it, the dog will think you're shouting at it for coming back, then when the dog does whatever you rebuked it for again, you'll repeat the procedure . . . and soon end up with a thoroughly confused dog.
A dog's mind works an awful lot faster than a human's, so we should bear this in mind when training. A good example of this is gripping (biting the sheep or pulling at their wool). If we see our dog grip, we're inclined to shout at it to stop, but it's really already too late! The dog's mind's moved on and he's considering his next move - which may be perfectly good. Scold him too late and the dog may well think it's this next move you disapprove of.