Why do my Keysight 4156A/B IV sweeps saturate around 10mA?

One possibility is that you are using the SMU SENSE lines instead of the SMU FORCE lines. Check your cable connections to the SMUs very carefully. When you are using single triax cable connections it iseasy to reverse the hookup. If the cables are connected to SMU SENSE instead of SMU FORCE, then the measurements will not be reliable. You should always use the SMU FORCE lines exclusively when making single (non-Kelvin) triax cable connections.