Shifting time it controlled by the bowden cable. It can be adjusted. It's connected to the linkages on the valve cover. Try adjusting that to get the RPM shift points higher.

As suggested you kickdown switch could be bad. You can unplug it and see what happens, you can also force the trans to downshift by clicking the switch with the pedal. When the pedal is all the way on the floor you can click the switch, which should cause the trans to downshift. If you get nothing then the switch is bad.

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