The LNB (the bit on the end of the pole jutting out of the dish) and the cabling can be affected by weather over the years. You may need to replace one or the other, or both, or the connectors between them. They can get waterlogged or corroded over time, for example.
It could also still be the position, don't dismiss it too soon because it looks fine to the eye and isn't swaying in the breeze. Wind over the years may have slightly changed the alignment of the dish, or the LNB relative to the dish.
Some tuners have a different sensitivity to others, can you borrow another box to see if that has it any better? It could be the box, though less likely.
---------- Post added at 16:32 ---------- Previous post was at 16:30 ----------
I see I was beaten to it as I was composing.
Chris' explanation is better.
