"Stuck" sliders (as in the LEDs do not move) would be due to turning off the "Vol/Exp" receive switch.

CP80 & Pad, Part 1 Edit, "Mod/Control" -> "Receive SW"

Notice "Vol/Exp" is turned off for this Part. This will make the slider unresponsive. Turn this on if you want the slider to adjust volume.

And (in addition - not your question, but related) ...

When sliders do not adjust volume like you expect, (usually) there's something else that is influencing the Part volume elsewhere. And by "elsewhere" I mean the "Mod/Control" -> "Control Assign" screen where source controllers are tied to destination parameters.

CP80 & Pad, Part 1 Edit, "Mod/Control" -> "Control Assign":

1) Touch "Display Filter", select "All"
2) Look at destinations searching for volume. Press "Page >" right arrow at top-right of screen to see more pages. Even if you find "volume" - keep looking through all the pages since destination parameters can show up more than once (possible, not very probable though). In this Performance, page 2 has Destination 7 as Volume.
3) Select the destination(s) you found. Here I'll touch Destination 7 -- exit out of the popup
4) Look at "sources". The source is AssignKnob 2. This is likely tied to Super Knob.
5) [PERFORMANCE] (HOME) - touch the Performance name at top and choose "Edit"
6) Navigate to "Control" -> "Control Assign" menu
7) Change "Display Filter" to "All"
8) Notice Destination 2 is "Part 1 Assign 2" - which is what is controlling volume in Part 1 as we saw in step 4. Touching this destination then dismissing shows the source is AssignKnob 2 (not the Part assignable knob, but the common level Assignable Knob 2 which is tied to superknob).
9) If you want to see the volume destination, touch the "Destination 1/4" box and move the DATA DIAL (jog wheel) to show "4/4" which is the 4th destination assigned to this knob. This isn't important to do - but it may feel better to see the right destination.

... this is what is also under control of volume. However, by default when you recall this Performance the volume offset is 0. So the slider should be able to work if the Vol/Exp receive switch was turned on.

Thank you Jason