When I add third-party VSTs, they work great the first time I open S1 after installing them to my plugin folder. However, if I close S1 and reopen it, the VST still appears in my instrument tab but doesn't open when placed on an instrument track. This happens regardless of vendor.

EDIT: This only happens with virtual instruments (drums, keys). Plugins like EQs and compressors do not have this issue.

What could be causing this issue?