Does IK Only Work With Rigs Made With the Unreal ART Tool?

This is really starting to bother me because I feel like I’m missing something simple. My rig has an issue where the IK doesn’t work for some reason. Is there some sort of “constraint” with the joints that’s making it not work or something because only Unreal’s ART rig works with IK instead of a custom made one(I tested with the same code.)

The result of my IK is simple - it simply doesn’t work or the leg shifts left instead of bending the knee, and even then the foot isn’t elevated or anything. It’s like broken or something. Please help because not even Maya’s Human IK rig works. What is the **** problem?

I have some screenshots presenting a case proving my concern. Here’s the custom rig PARTIALLY working. I just don’t get it… As you can see for some reason the right foot works, but not the left.