Answer

There are two explanations: 

• In the case of a wireless device this may be because the mixer's Remote Control Access settings for the device are not set to allow the device to control the selected mix. 
• The second possible reason is that the missing faders are, by design not available for the mix being viewed. For example, the Aux Output Faders 9-16 are not shown on Aux Mixes 1-8 because these outputs cannot be mixed onto Auxes 1-8.