There might be a couple of reasons why. First, is that the sensations may be too strong therefore she is moving the unit away from her clitoris somewhat and this could be enough to prevent the building of sensations she needs. Second, which goes along with the first, the vibrations may not be mimicing what she does by hand so she cannot reach orgasm even though there is more strength and intensity to what she is doing, and ultimately more enjoyment. This is not unlike giving a guy a blow job and him not being able to climax from the effort.

I offer this cautionary note: As enjoyable as any vibrator may be, a person should not rely upon it all the time because it is quite common for a woman to use a vibrator and then not be able to climax by hand because she has conditioned herself to the vibrator. Neither of you want this to happen, so use the unit and enjoy it in moderation. When she climaxes from her fingering or yours, just know that the vibrations that preceeded made the orgasm and the build up just that more enjoyable.