How do I Force my Wireless Card to use Mode 802.11n?
It's possible your signal quality is too low to support 11n communications. 11g has less throughput but more reliability than 11n. If you were really far out of range, your machine would switch to 11b. I have had machines that do this and I love it. I'd rather have slow network than no network.
To fix, I'd get an external antenna for your router, one with some gain to it. These can be had cheaply from various online retailers, including Amazon.