Why are Logic Pro samplers not playing sound in certain channels?

I am having an issue and I'm going nuts trying to get this working... I have a concert with several song's in. The channel strips show activity on the virtual keyboard. But no sound is coming out, and the meters on the channel strips aren't moving. When I duplicate the channel strip the new one plays. And then I duplicate the old one, but after a few seconds, it stops playing. And if I switch to a different patch and come back, it won't play. Certain instruments, such as studio horns are fine. But the samplers don't play. I have a Tyrell sampler I downloaded and all of its sounds play fine. But the samplers that come with Logic don't seem to be working properly. I don't know if it has anything to do with these samplers, but I am very frustrated. I'm hoping someone can help.

[Re-Titled by Moderator]

Original Title: Channels are silent