If you are using the toggle (trigger) switch to select pinpoint all metal, try using the first discriminator knob to click into all metal and leave the toggle switch in it's middle position. This all metal mode is little slower on the retune than the pinpoint.
Tejon is sensitive to the coil not being lowered and raised with the coil level. If you are moving the coil up and down by just bending at the elbow, then there is an arc in the swing that adds a little to overshoot and makes it more difficult to get it ground balanced.
The Tejon manual references the issue as follows: "NOTE: Please remember that the coil must be lifted straight off the ground. Swinging the coil in an arc, like a pendulum, will cause false readings and will result in an improper ground balance."
Try pausing the down / up motion with the coil near the ground and not just pumping up and down like is easy to do for setting up the Vaquero. Try adjusting it positive and just doing a down stroke then holding the coil steady at the end of the down stroke. Lift it slowly then push it straight down again and pause. Pay attention on what the threshold does on the down stroke before it starts retuning to the ground when the coil is held at the lowered position. If it raises on the down stroke, turn the ground balance counter clockwise some. Repeat until it does not raise on the down stroke. If you go too far, the threshold will null (go away) on the down stroke.
As much as I like the Tejon, the retune on the trigger activated pinpoint is a little too fast for my tastes.
Cheers,
tvr