I had this once before, and it was to do with rasbian disabling ssh until I had changed the password (in fact I think it came up with a prompt to that effect on the console).
Also, if you are using a multiband wireless router, some (notably BT Home Hubs amongst others) do not allow traffic to cross from 5G to 2.4G network or vice versa, but allow both networks to the internet, they are treated as if they were 2 separate internal networks.