It has nothing to do with online/single player. Some G29's have paddle shift issues after long use. It happened to mine also.
Basically, the button that the paddle shifter "clicks" has a soft rubber landing that the metal paddle contacts. If you look down from the top, you should see what I mean.
Anyway, that soft rubber wears out over time and the shift strike isn't long enough.
I fixed mine by cutting 3 squares of electrical tape and applying them to the paddle where it meets the button. 6+ months with no issues.