i think you have 2 options here,
1) maybe the indexing isnt really spot on and you need to adjust the cable tension so there is less cable tension. try moving the barrel adjuster a bit and seeing if you can find a place where the mech moves both ways happily
2) the indexing is good, but for some reason the spring in the mech isn’t pulling the cable properly. This could be due to worn outer \ frayed inner, the inner cable just needing a clean and lube, inner cable is kinked somewhere inside the outer casing, or maybe the cable routing is causing a problem and some of the bends in the outer cable are a bit sharp.