To launch a successful shooting attack requires a degree of preparation as most weapons take a long time to load or aim. As a result, a model that wishes to make a shooting attack must have had a relatively uneventful Movement phase. The most common reasons a model cannot shoot are:
- They marched in the preceding Movement phase. 
- They rallied or declared a charge. 
- They're fighting in close combat (and therefore for their lives). 
- They're fleeing – such models are far more interested in saving their skins than firing their weapons. 
- The model is under the influence of a specific spell or special rule that prevents it from making a shooting attack. 
This is, of course, not a comprehensive list. Other situations will doubtless arise, due to spell effects, army special rules and other factors. These will be explained by the relevant rule as and when they occur.