I agree the temps are almost certainly too high. I don't change the temps of my heat tape ever, but because I live in MO the weather changes and so my ambient temps do as well. Even my very first year I had locks, and I had no clue what I was doing at first. If your climate doesn't change like mine does, it might be necessary to change your temps on a set schedule.
I find it really hard to understand you haven't had any locks ever, something must be very wrong for that to happen. You don't specify when you pair your snakes. Is there a certain time of year you try, or have you tried at different times of the year to see if that was the reason ?
I understand what works for someone else may not work for you, so I hope we can help you figure this out.
Gale
well I have just being doing it continuously all year long because I didn't know when it was going to happen since I had the temps the same all year long.