Have to manually connect everytime...

When I turned my PB on it would always automatically connect to my home wireless network (after I'd entered the password etc. of course). But now I have to manually select the network everytime I turn it on - I've played with a few settings to no avail and this problem does not occur with other networks that I've joined (at my folks for example). It's not a biggie but I'm scratching my head and it's just a little frustrating. Any ideas? Cheers