The detent plunger may need to be screwed in one or two clicks, or, it may be worn and rounded off. If worn, sharpen it with a file. Be sure to count the number of half turns when you remove the plunger so you can put it back on reinstall.
If that does not help, your ratchet mechanism may need adjustment, for this you should get Pete Snidal manual from Classic Motorworks or Nfieldgear.