Hoping someone can recommend an easy fix for a 1990' Shimano 3X7 Altus thumb shifter I have that shifts down into the smaller cogs easily but then catches a little as I shift back up into the bigger cogs. I 've replaced the cassette, chain, shifter cable but still not super smooth.

Is it something to do with the shifter itself or the rear derailleur? there is no problem shifting with the front derailleur and I haven't made any changes to that.