I had this issue last year with my 12 tame lambs... this year we've got slightly more, 17 in total, and they seem to be keeping as a flock more and touch wood haven't escaped yet. They still come to call but are not quite as tame as last year, maybe because they were on an adlib feeder which took some of the hands on contact away.
If you're really stuck I guess you could put them through market... it's a bugger when they won't stay in the field, I moved mine to a new field yesterday and told them if they escaped then it was a 1 way ticket to market next week
