The problem with this kind of issue where no one is a professional is that the cause could be something simple, such as a setting, especially something related to routing. Or, it could be a very complex problem or system failure. However, as Peter mentioned, we will need a lot more information to be of help. Besides the info requested by Peter, we need to know how the problem presents itself. Saying, "the centre 8 channels have never worked since they got the board" tells us nothing. Here's more of the kind of thing we need, for example:
1. What kinds of inputs will be connected to the center 8 channels (channels 9-16)?
2. Has someone connected a microphone to those inputs and no sound came out?
3. Was some other device connected to provide input, such as a disc player to supply music, or some musical instruments?
4. How was it connected (through what kind of connector)?
5. Is the person who set up the currently working channels available to set up 9-16? If so, what has been tried and didn't work?
There are many more questions I could add, but a mixer of this sophistication being operated by people with little or no understanding of its capabilities and configuration is like asking a layman to pilot a jet with no training — the flight is quite unlikely to go well.
So please understand, we want to help, but we can only do so if we can get sufficient information to understand the problem and what has been attempted to fix it. And on your end, at the very least, it'll require someone who is willing to read the manual and learn the basic concepts; better would be if you could find someone who has at least some basic familiarity with the concepts of any kind of mixer.