sometimes when I try to use 16 levels to "assign" one sample over all pads I´m not able change the original pad (default = 4). The workaround then is (this is pretty sick I don´t know how I found this out) to first assign the sample to the slider then "unassign" it and BOOM all of a sudden I can change that original pad again.
This is a longshot but maybe it works for you too
Yeah, I found that you have to be on the main screen to use it and even still sometimes it freeze's so try going to other menu's or play a sequence then stop it then see if it works.