Remember that the 802.11b/g/a specifications have a certain distance requirement. Along with that distance requirement, you have to factor in other factors that could cause interference of the signal. These factors can be walls, microwaves, 2.4 GHz wireless home phones, environmental factors (if you're trying to access it from the outdoors), interference from other wireless devices (specifically the 802.11n ones), etc.
Also, the placement of your wireless router/access point is important as well. If it's placed somewhere on the floor, you won't get a good enough signal as if you've placed it in an elevated plane. Remember that the signal radiates from the access point in a bubble-like fashion, so take into consideration where you place the router/access point so that you can maximize your signal strength even given the amount of interfering factors you have to deal with.