Siri won't let me finish

When I try to dictate a text message into Siri I have to keep talking without a pause. If I pause--even for a fraction of a second--Siri cuts me off with "Ready to send it?" Is there any way to adjust this? I really like to dictate my text messages, but this issue really causes problems. I should probably add that I use an iphone 5C (which is otherwise fine for my purposes).