Sounds like a missing or greased up friction washer. But sometimes the needed friction setting makes shifting harder. If your frame's rear is spaced at or can be cold set to 126mm then think about a 7-speed indexed upgrade. It should be possible to find a used Shimano DT shifter and a compatible derailleur. The indents do a good job of holding the setting without a lot of lever effort in shifting. DT index shifting is very reliable and easy to use as long as you are on drop bars.