System Workbench for STM32

Good day!
I start with STM32F4-DISCOVERY and CubeMX. First I write code that generate sinus using DMA and DAC.
This work good. Next I need to trancieve message throutgh USB when generation of sinus go.
CubeMX give USB_CDC library and I use it. Problem is when I create Transmit function and put it in main loop
generation of sinus not started but Tramsmit work properly.
What can you say about this?