Well, are you making sure your entering the correct digits?
Also, remember that the Souvineir will not be in your items - it will be in your soveniers.
And check that you have the right username - a username is somethign you had to enter in your tamagotchi toy once you bought it. It lets you explore tamatown. Make sure you are using the same username in tamatown, as you are on your toy. To get to your username, on your tama go to the first icon. Then, keep pressing A until you reach a screen that says 'Username:' and it will show your username.
Otherwise, I'm not sure.
PM me for tamagotchi help.