I had this problem for a year before solving it, the information is repeated in this forum under other topics, but you have to dig.
Symptom: The right lever shifts down, into larger cogs, but not up, at least not all the time. Reversed with the left shifter, it goes in the big ring, but won't snap down to the small ring.
1. Your problem is not wear on the ratchet pawls most likely. It's the years of WD40 you've been spraying in there, which leaves a gunky residue once the solvent evaporates. Don't use it for general purpose lubrication! It is a good water displacent, but a poor lubricant or degreaser.
2. Get a can of "Brake Parts Degreaser" or anything similar. The key ingredient you want is Trichloroethane or TCE. This is very bad stuff, but you won't be using much. Still: Don't breathe it, don't get it on your skin or anything else you like, and if you do, wash with soap and water right away. It will strip the fat out of your skn and dry it out painfully, just use hand lotion after washing, or better yet gloves before and during.
3. Spray the degreaser forcefully into the shifting mechanism in as many places as you can. The stuff will drip down and make a puddle on the floor, that's OK. Work the gears up and down, spray again, then blow it out with compressed air. You can also use canned air from an electronics or office supply store.
I suffered this problem for a really long time on a 15 year old bike, in cool weather it wouldn't shift at all, and I'd have to pick a gear for the day. One round of the above completely solved the problem. When I took it out, the shifts were painfully crisp -- there was no lubricant at all!
4. Relubricate. NOT WD40! Light machine oil is the ticket. Spray it in, or pour it in and blow it around with compressed air again, then wipe off the excess. 3-in-one oil is fine, but there are lots of good light machine oils out there, just not WD40.
Thanks to the many posters who went before me and saved me an expensive and unnecessary upgrade.