I had this issue with Multibit several times. I think it is because of a bad internet connection while sending. The transactions is shown in the client as sent but when you look it up on a block explorer the transaction is not existing. It wasn't just not spread with the network. My way to solve it: Close the client and reopen it. Sometime the transaction is vanished so you need to send it again. Sometimes it is shown as sent and it is also noticed on the block explorer.