Twenty years is a tricky cutoff. If memory serves, that makes you a post Hyperglide mechanic.

This isn't a putdown, byt a reference to what you've worked with and your expectations. For example, if the middle ring isn't gated, you might be looking for better performance than possible.

Other factors may include, the condition or the ring, the tooth profile and/or the separation between the middle ring and outer which can make it difficult to bring the chain over to snag a tooth without dragging against the face of the outer. Wear in the inner cage plate might affect it's ability to wedge the chain upward, or the old cage may be more flexible than newer ones.

There's also the off chance that the middle ring is flipped over (inside out), or there's unfavorable facing so the inner wrong part of the chain meets the ramps or pickup pins.

All that is besides a sticky cable and/or some kind of worn shifter issue.