Joined: Jan 2010
Posts: 9,797
Likes: 1,761
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
It sounds like this might be an STI-type lever.
When the OP "pushes the lever slightly foreward, it will catch and shift".
This tells me that the "foreward" movement might be causing the OP to move the lever a littl more slowly, which will usually allow a gummed pawl to catch the ratchet wheel.
These problems always show up with the arrival of colder weather, and the repair suggestions for this symptom are all over the map, from flooding with WD40 to immersion in hot oil.
I prefer to remove the hoods and immerse in hot oil, as this has never failed to give a very long second service life in my experience.
The still-hot shifter will need to be whirled at the end of rope or bungee to expell excess oil that would otherwise tend to leach out into the rubber lever hood.