What low gear do you have? My mtb has a 22x34 low gear. I can get up a steeper pitch, even with some rough stuff, than I would try on the road bike which has a 34x29 low.

I also have a fork I can lower a bit on my mtb, and I lower the saddle from where I like it on the road bike for steep climbs. These adjustments seem to help climbing technical trails.
Also, I run the front tire softer when climbing trails, seems to help keep it from bouncing off rocks and makes it easier to hold a line.
Sometimes I get to where I can't seem to make the front wheel go where I want it to go on a climb and this problem gets worse when I get tired.