You're probably running out of polophony!
Experienced users would be a little more helpful if you were to explain your issue a little clearer too. What voice version are you rocking, is it a unison patch you're running, you know, standard kind of stuff.
The answer above has hopefully cleared up the answer for you, basically you don't have cubase inside your new toy and a little read through the manual would of helped you out there.
Also bear in mind that users here are not at liberty to help you out, though this group of people are very helpful, a little patience and I'm sure you'll get your answer.