dishwasher suddenly doing a half-assed job

[Mr Downtown]

I've never had any issues with my dishwasher (a 25-year-old Maytag) until this month. Suddenly it's leaving food on things, especially in the top rack. I've opened it at various points in the cycle and things look normal, though one thing I can't think how to check is the central "spray wand" that would extend upward (by water pressure, AFAIK) through the top rack and spray water all around. Nothing seems to be blocking that, but I can't think how to test its functioning, since it would drops back to the resting position as soon as I open the door.

Anything sound obvious that I could check?