I've tried climbing on the default unity plane with different physics material - the same result.A few things to try:
- Check the collider on the slope itself. If it's a complex mesh with a lot of tris, there might be bumps etc. that the character struggles to smoothly walk up. You generally want smooth, straight slopes as much as possible
- Try adding a frictionless physics material to the slope (and set the friction mode to "minimum")
- Try increasing the "Slope limit" on your UltimateCharacterLocomotion component (under "Physics")