2 fantastic looking dogs there.
Can sympathise about the Lexie, sounds like she's got the strongest traits of the colly and the strongest traits of the Terrier.
We've got a Jack Russell that was like that, spent weeks with a treat bag on my belt and i'd keep calling her back and giving her a treat.
She got to the point where she'd come to call 9 times out of 10, but i found on the 10th i was sounding a bit stressed and irate.
So i then started calling her back and giving her a treat while trying to sound irate and angry.
Worked great, she comes back when called every time now.
Took weeks and weeks of training though and some very odd looks from folks as i tried calling her back sounding angry for no reason lol
Cheers
Mark