I have shimano 600 shifters on my off-season bike. When I am shifting from a smaller to a larger (against the spring), the STI lever does not "catch" anything. If I wait a while, it will eventually engage and I can make the shift. This happens on both sides and seems to be worse on colder days and often at the bottom of a hill.
It seems to me that there must be a ratchet mechanism that is sluggish or something.
Any ideas, experiences, suggestions?