When your bishop makes the queen the target of a double attack, you can't generally count on a fork or other nifty maneuver to save the day when your opponent replies QxB. So typically a fork against the queen only works if the bishop has protection. Here are a few examples of how this looks.
In the position on the left, White looks at his checks and sees that Qd8 forces Black's king to h7. He looks for his next check and finds Bf5, which forks Black's king and queen—but fails because Black simply plays QxB; by then, White's queen no longer is on the fifth rank to supply protection. The fork would work fine if only the bishop had a guard. Ah, but White can have it both ways by giving the check with a different piece: Rb8. This likewise forces Black’s king and queen onto the same diagonal, and this time White wins with Bf5+.
Notice that the problem starts with Black threatening checkmate by playing Qxg2. In the face of this threat it would be natural for White to think defensively, but as we know, sometimes the best defense is a good offense.