Q-sampler not loading instruments

When I open up a prior session - sometimes my Q-Sampler won't play.

Sound still runs through it if i click the small play button in the sampler and also when i press buttons on my keyboard, but it wont register midi events in the piano roll. If i click on the keys in piano roll no sound comes out and if i click in notes with a mouse no sound comes out.

Not all instances of q-sampler within a session will do this.

Any ideas?