It sounds as if your extender is not reliably maintaining its connection and is dropping off the network. When that happens, your computer is configured to log onto the network and doesn't care wether it gets the network from the router or extender, its job is to maintain the network connection regardless of the source. When the extender loses its network connection the computer quite correctly reconnects to the strongest signal on the network it can find. Once a network connection is established the computer does not continue to search for alternate sources.
So if your extender is losing its network connection, your computer will automatically seize the signal from the router and when the extender reconnects to the network your computer will not reconnect to it. There is no reason for the computer to look for another network source unless its existing connection to the router fails.
Have made that clear as mud?
I know what I am thinking, but I'm not sure I am saying it clearly.
