1920-S Bison Nickel - Weak Strike Or Worn Die?

I don't think the strike is weak. There is nearly a complete rachis and calamus on the second feather and the date and lower ribbon are bold. The horn is sharp, along with the buffalo's beard, neck, and legs.

Your coin's die bears several similarities to what became the 1920-S FS-401 2-Feather. I wonder if it could be an earlier die state of the same die pair. There are three very light die cracks that seem to be on both coins (beneath the buffalo's head at the rim, curving between the buffalo's left rear and left front legs, and between the buffalo's head and the "U" of UNITED). The extensive die polishing that removed the third feather on FS-401 is a box-shaped polishing, in the same general shape and location as the polishing on your coin. Both coins have the top of the mint mark abraded in similar fashion. Both have LIBERTY heavily abraded toward the rim in similar fashion. On the 2-Feather variety, additional die polishing covered parts of the small die cracks and finally removed the third feather, which has been weakened on your coin by polishing, but is still mostly visible. IF this is an earlier die state, the heavier die polishing on the 2-Feather would have been a later polishing, which is why your coin doesn't show that level of die polishing. That could also explain the relative strength of the "19" in your coin's date versus the general weakness in the "19" on the complete 2-Feather.

On your coin, I'm assuming that the parallel marks near the buffalo's rear legs and tail are raised, rather than incuse, and reflect a fairly strong die clash polishing shortly before your coin was struck. Multiple rounds of die polishing are common, especially at the branch mints.

Regardless of whether your coin is an earlier strike from the die pair that eventually became the 2-Feather, my read is that your coin is late die state, with extensive die clash polishing. On your coin, the buffalo's head, "F" of FIVE, and "S" of CENTS are just beginning to merge with the rim. Part of that is the die state, but most is circulation wear. The tail has merged. For most of us (but not PCGS), that places it in the VF range. There is more wear on the buffalo's head and upper back than I would like to see. I would personally net this out as VF-25.

The reverse is showing die flow lines from Die Deterioration. Other that that, I have nothing else to add.