When VCs tell you that they're not interested, they may provide you with little insight about why they turned you down.  The level of transparency provided in the rejection email varies based on both the VC's personal style and tactical considerations.

With regard to style, some VCs are less direct than others. As a result, they may avoid a direct explanation to be polite. They may also feel a need to hold their tongue if you were referred by a friend or important business contact. VCs know that most entrepreneurs take VC feedback personally.

There's also another key issue; giving feedback can be time consuming.  Not only does it take time to craft a thoughtful email, but also they often lead entrepreneurs to offer more information or ask more questions.  Entrepreneurs often view feedback as an invitation to engage in a dialogue.  Unfortunately, VCs don't have time to engage in dialogues with thousands of entrepreneurs.

If a VC doesn't provide sufficient feedback, ask for it and then just listen.  Once you are done listening, if you feel you could refute the VC's stated reasons for rejection, ask if the VC would be willing to hear your arguments.  The answer might be 'no'. That doesn't mean that the VC isn't open to your points, but it may mean he's aware of them and/or has other reasons for rejecting that he's decided not to share.