Generally speaking there are several factors invoved:
the level of teammates plays the role --- if you have the only one good player on your team, that guy will take a ton of shots. More shots usually means less field goal percentage.
The level of opposing defender has a huge impact.
Another thing:
For guard it is important to have good driving, handling and passing. In a weird way from my experience I can tell, that guard with good inside scoring will have higher percentage.