Originally Posted by
Aubergine
This is a common Shimano problem. The mechanism inside the shifter gets gummed up and the pawls no longer catch or release as designed. Take the shifter cover off and clean the moving bits with triflow or other light oil. Watch how the pawls work and you should be able to spot where it is hung up.once that part is loosened and working freely, your shifter should be fine.
Huh...I have a couple sets of M590 and M570 shifters that are older and do not have this problem. Is it limited to lower level stuff, like Acera or Altus in this case? I generally go for Deore level for my own bikes, so I don't have experience with much below that.