low signal on airport extreme/express

i have an airport extreme base station on the floor above me as the main router. To extend the signal to my floor, I have an airport express in my room. I friend installed kismac to guage the signal strength. When the airports were initially set up, kismac read the extreme signal around 40 and the express signal around 60. Now however, both signals have significantly dropped, causing connection problems. Is there a way I can bring the signal strength up? Also, kismac says that my network is "probe" instead of "managed". Would that have any effect on the signal strength?