Can not enter DFU mode when DFU code combine with app code

[Treacy Yao]

I use cube to generate DFU code and test its function. It is fine(device can enter DFU mode).But after I download APP code to 0x0800A000, then test again. Device can jump to app successfully but device cannot enter DFU mode. Device descriptor request failed appear. I don't know where is the problem