My Bianchi has Suntour Cyclone derailleurs. Smooth, fast to shift, quiet, light. But the RD has a problem: It doesn't want to go onto the smallest cog (smallest of 6). The stop screw isn't the problem, the parallelogram spring is. Or springs are, for there are two of them. The cable and housing are newish Jagwire and operate smoothly. The housing is a bit stiff so it appears to be wishing it didn't have to bend quite as much as it is needs to to enter the back plate of the parallelogram. But the biggest problem is simply that the springs aren't very strong.
Any thoughts on what else I can do to help this? Disassembling the parallelogram doesn't look feasible.
I noticed this problem when I first assembled the bike but managed to overcome it with lubing, bending, coaxing, and pleading. The problem has returned. I don't have many occasions to use the small cog, but I noticed it when cleaning up after this weekend's ride. No point in having a 6-speed FW when you can use only 5.