Try running the game as administrator.
If that doesn't work, you could try running the game using the eDuke32 source port instead of DOSBox. This will allow you to run the game natively in Windows and also includes a lot of (optional) extras including a true mouselook and higher res graphics. There's even an add-on that replaces enemy sprites with 3D models if you want.
I ran eDuke32 with all the original graphics, the only thing I added was the true mouselook.
There are many posts in these forums detailing how to install eDuke32. For example check here: http://www.gog.com/en/forum/duke_nukem_series/eduke32_help