Adopted two sheltie brothers from the same litter. We got them a month or so ago. They were never socialized and were 7 mos old when we got them. They are very close to one another and werenever leash trained. They are afraid to leave our yard. When i try taking one of them out, THEY PULL BACK and slip off the collar and try to run back into the yard. Don't wantto pull the collar too tight and they even manage to struggle out of a choak collar. Had many dogs and never encountered this problem. What do i do?