Invoices not showing up in Accounts Receivable

[Ben]

Hi, I'm new to GnuCash (and book-keeping to be honest). I've worked 
through the documentation/tutorial (which was excellent) but I'm now a 
bit confused.

I've added several invoices, all of which appear in Accounts Receivable, 
but only *some* of which alter the balance in AR.

I can't for the life of me see what I've done differently with the 
different invoices. I also created a new file and tried creating some 
test invoices, but even worse, I see nothing in the AR balance at all 
(just 0.00).

Is this a problem with Vista (I've noticed that the date pickers don't 
work, and I always have to pick Accounts Receivable when posting an 
invoice or it complains that no account is selected) or a common 
misunderstanding caused by my extreme stupidity?

Any tips/suggestions or comments about what further information I can 
provide are *very welcome*,

Thanks,

Ben