Windows phone doesn't save characters typed into the user name field when you're signing in to a site. If your user name for a site is not a normal English word for example your first and second names together you're stuck when trying to use the swipy keyboard
Here's a workaround.
Start Word and type your user name. Tap it again to select it and then tap the word in the word flow bar with a + symbol after it. Your user name is now saved and you can use the swipy keyboard to fill out your user name fields on sites where you need to sign in.