If "automatically load samples" is not checked in the SC editor "preferences" dialogue then the sample will not actually be loaded in RAM and therefore will not play. A way to determine this would be to go into the sample map window and see if the sample names are in italics. If they are in italics, then this means they are not loaded in RAM. Use the "Load Samples" command, or close the Bank/Instrument, enable "Automatically Load Samples" in Preferences, then reopen the Bank/Instrument.