I'm not sure why this occurs, but for some random reason any virtual instrument that plays chords stops working in Studio One v.4. My solution to this is typically save the Song and then exit out of S.O. and restart the application, then I can hear that track again. For now I'm using the internal audio processor on my MacBook, since I'm only creating instrument tracks. Any pointers you may have to resolve this will be greatly appreciated. Thanks and have a wonderful day!!!
MacBook Pro (Retina, 15-inch, Mid 2015), 2.2 GHz Intel Core i7, 16 GB 1600 MHz DDR3, macOS Mojave 10.14.6 (18G103)