Chat windows isn't displaying properly

I have installed the Chat plugin but the chat box doesn't display properly. It's fine when it's not being used. It appears as a bar in the bottom right corner that says "Click here to chat!" just as it should. But when you actually click on it the box doesn't open correctly. You can only see the very top of the box. I've attached a screenshot showing how the chat box appears when it is opened (you'll notice that "Click here to chat!" has turned into "Chat" which indicates that it is in the open state).

I thought it might be in the settings, but as the other screenshot I attached shows my chat box should be 400×400.

What can I do to get this working right?