The short version is I have an IK setup on my rig. The legs work mostly okay, except for the knees. The knees will NOT bend to find a solution to the IK problem. The arms, set up in a similar way, will bend correctly at the elbows to find a solution.
If I lift the foot pad goal up slightly, the knees will NOT bend. However, if I change the constraints on the knees and allow them to bend backwards, they WILL do that to solve the problem. They won't bend in the correct direction.
I thought this might have something to do with the rotation of the bones, so I tried rolling them with no luck. I tried adjusting the position of the knee pole, but that didn't help either. I can't seem to figure out why the IK solver is so opposed to bending the knees to solve a problem. The issue happens on both legs.
I do have a small bend in the knees, so the related issue (Similar issue: IK-rig doesn't bend when I add elbow-target/elbow-pole) doesn't seem to apply. I can't share the .blend file yet, but I'm working on getting permission.
EDIT: I've tried every permutation of rolling the bones, changing the pole angle, and knee bend limit.
The Leg Setup:
Leg Setup with Axes Showing:
Solution Not Found:
Visible Constraints in the Knee:
Relaxed Constraints and the Bad Solution:
EDIT: Adding emphasis.
EDIT: Adding more images.
If I turn off the knee constraints, leaving only the pole bone, the hip twists backwards and the knee points away. That might seem like the pole rotation is wrong, so I tried reversing it... (See next picture.)
When I reverse the pole angle, the hip twists around 180 degrees to compensate. The knee STILL bends backwards, but points towards the pole.