I can reproduce exactly this effect when the physical image-file is not available
My files are "present and accounted for" when this happens, so that's not the issue in my case.
Here's the sequence. After I check the soft proof check box, choose the appropriate profile, check "sim paper & ink" - all is well right up to making the first slider correction. Slider moves fine, dialog pops up asking re create proof copy, when I create proof copy it does so - and sliders gray-out. I need to uncheck then recheck the soft proof check box to get the sliders back. Disconcerting until I figureded out how to "get them back," - now a mere inconvenience.