Target platform? Atlas texture size? Target quality setting? Missing info there...
Unity can sometimes force your textures to get imported at lower size than the actual texture, which screws up NGUI. It may happen due to quality settings being set too low, or to target platform not supporting large size textures. For example GLES 1.1 / ARMv6 targets are limited to 1024x1024, and many mobiles are capped at 2048x2048. Only newest mobiles can go up to 4096x4096.
It's advisable to keep large textures outside of the atlas. Atlases are ideal for textures that are under 256x256 pixels.