Airport Express Weak Signal - 3 feet away

Since I installed the last Airport update, I've been having a fluctuating signal which most of the time is weak (6 bars out of 15). Sometimes it gets to the 15 bars, but only for a short period, and then goes back to 6 bars. I tried changing channels but I still have the same problems. My MBP is around 3 feet away from the base and there's nothing in between that could obstruct or interfere with the signal (only a regular wooden table). When I use interference robustness, the signal improves a bit, but it's supposed to be way stronger since I'm so close to the base. On public networks I get a pretty strong signal, showing that the problem is not in my computer.

It's pretty annoying browsing the internet with such variations in the signal (sometimes I get stuck while the browser - Safari - is loading a very simple and light page).

Any suggestions/ideas?

Thanks.