Trans won't shift up

I have a 91 Buick Skylark, auto trans, 2.5liter. It starts just fine. Takes off immediately into first gear. Also has reverse. But, it won't shift into 2nd gear or higher. Anyone seen this problem before, or have any suggestions. I been through several repair manuals in the last few months, but can't put my finger on exact cause.

I would check the throttle valve in the throttle and on the trans. This will cause delayed upshifts; it controls the trans based on fluid pressure. If it is out of adjustment, a late shift could be mistaken for a no-shift. Ray

I've checked the throttle valve a few times already. It seems to be adjusted, and working just fine. I do believe I have more than a delay. I can drive the car for a mile @ 20mph, without it shifting up. TYVM for the suggestion though.