Assuming you have the first run which have a sort of clickety-friction shifter,
I would start by adjusting the cable at the derailer. Get it in a gear and look
at the pulleys. If they're directly under the cog, turn the adjuster on the
derailer clockwise enough to move the pulleys slightly right
I'm having problems upshifting the rear derailer on my newish Clem L. I have
the stock Sunracer friction shifters. With less than 300 miles on it, mine is
slow to upshift into higher gears when heading onto level ground--sometimes
slipping through 2-3 gears. No slippage downshifting going up hi
