NurnNotice also mentioned tires, which is a big factor in traction whether standing or sitting. There's a section on a trail I recently explored that I just know I could get up if I swapped from my Black Panthers at 28 psi back to the Fire XC Pros at about 25 psi. I'm not a fast rider, so being able to stay on the bike over technical stuff is a source of pride for me.
Urbanknight, is your terrain very rocky/rooty or loose? I also run a Panaracer Fire XC Pro on the back wheel (usually on a Hutchinson Python but now too pricey to buy again), and I am having traction problem on steep uphill of loose dirt or on exposed granite rock. I run around 36 psi, which maybe a little high, I dunno, but I feel that for the rest of the terrain, 36 is good (tube tires, not tubeless), and I'm on a hardtail, so tooo high is hard on the multiple roots, and too low... well. And that Fire XC Pro tire feels like a real tractor tire, but it's a 2.1, I should try a 1.9 maybe? My plan is to swap it for a IRC Serac XC which I have in front right now.