It's difficult to provide you any real advice without knowing anything about your service/product, or what a 'signup' means (is it a free version of your product, a signup to your mailing list or something else) but generally speaking it can be any of the following reasons:

1) People sign up to get something that they want, but don't need your product, or want to pay for it.

2) Your clients deem the free version to be sufficient and don't want to pay for the paid version.

3) People sign up due to curiosity but the value isn't there for them to purchase.

4) You haven't done a good enough job communicating your product's benefits to your signups.

5) You lack sufficient "calls to action" that would convert signups to paying customers.

Again, this is a gross generaliation, as no-one will be able to give you any specific feedback without having more information about what you're selling.

Hope this helps.
Bryan