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 – weapons will have been shouldered and ammunition secured while the unit advanced.
They reformed, 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. Fear not! These will be explained by the relevant rule as and when they occur