By default, many solo instruments in the Garritan sample libraries are set to not play more than one note simultaneously. This feature, called Auto-Legato, can be disabled in the ARIA Player standalone, or be switched off using a controller. Please note that for some instruments (such as those in JABB3), turning off Auto-Legato will not enable polyphony.
In the ARIA Player, load an instrument in a channel and click on the Controls tab. In the lower left, you'll see that Auto-Legato is enabled. Click the button to disable it. This feature is assigned to controller 102, which can be toggled in a notation or sequencing application. For example, in Finale, you can create an expression to set controller 102 to 0 ("off").