I'm guessing you are trying to access unsecured networks that were not configured by you.
Many wireless AP (access points), seem to be unsecure, but infact block all unwanted users (using a feature that allows them to set which MAC addresses are allowed to connect). In that case, it will look like you are connected, when infact you cannot accttualy use the net.
Another reason might be the distance of the AP. If you do not have a strong enough signle, you will not be able to connect (it takes a stronger signle to connect, than to surf after you are connected).