I'm guessing a couple of things here. First, I think you probably have your terms mixed up. An 'upshift' is shifting to a higher/harder gear, so physically to a smaller cog. A 'downshift' is shifting to an easier/lower gear, so shifting to a larger cog. While I mostly agree w/
Iride01 that we need more details to provide useful help I thinks it's probably a cable/housing issue. Friction from worn housing, dirt/corrosion...something like that. I'm not sure why he uses the term 'holdfast' when cable anchor is a lot more widely used and easier to understand for most people.