first the solution: CMD-click (CTRL-click on PC) the MALLET TYPE button to purge all, then click it again to unpurge all articulations will play now.
then the explanation: In some cases, most notably after a resave of the instrument if some articulations were purged at the time of the resave, the articulations would show as unpurged but really be purged - the samples are not loaded into RAM so they will not play, but since the script thinks they are loaded, it will let you select them as if they were.