8+ cassettes are wider than 7- freewheels and do require more RD travel. Some older systems just don't have the cable pull to make it. Or it's possible your RD doesn't have the throw to get all the way to the bottom cog. I don't know about your Suntour stuff specifically, but for instance an NR shifter has a pretty small barrel and a NR RD has a pretty high ratio to match, so if you hook up a NR shifter to a modern Shimano RD you might have trouble getting it to move far enough. I have this problem on my Super Sport which has a Shimano RD and a shifter that was made to go with a Huret. I have juuuuust enough cable to shift it over 6 speeds. With an 8-speed cassette I'd have to give up on the outer gear.