1999 Lockup torque converter problems

I have a 1999 with the 3spd auto with the lockup torque converter. I bought the car a month ago. Soon as I bought it I got a check engine light. It was the lockup not working. Then 2 weeks later the tranny started slipping in 3rd. I took it to be rebuilt but they can't get the lockup to work. Everything is new in the trans. The computer is telling the selenoid to activate the lockup but its not doing it. Any ideas?

Wally