Pizzicato Is Muting My Audio Playback

I am arranging a string quartet for my instrumentation class and I have a section in my score where I have all the strings in pizzicato, but for some reason, when I put pizzicato on the viola, it mutes the playback of just the viola. The pizzicato works great on the violins and cello, but for some reason, it mutes the audio for just the viola when it is under pizzicato. As soon as I put arco over it, the viola audio works fine again. Is this some sort of bug? Any assistance would be greatly appreciated.