Physical tension is the biggest hurdle. Because the sensation of needing to "squirt" often feels identical to the urge to urinate, many people instinctively tense up to stop it.
Many people require simultaneous clitoral stimulation to reach the level of arousal necessary for the Skene's glands to respond.
Using one or two fingers to apply firm, rhythmic pressure to the anterior wall (the front side, toward the belly button).