Send Button Disappears When Composing Message

I have the strangest problem with my Mac Mail on my MacBookPro.

When I'm composing a message, sometimes the 'Send' icon (the paper airplane) is not available the message I'm composing. I've found a workaround by saving the message as a draft, and then opening it back up again, which somehow restores the Send icon in the message. Also, sometimes after closing mail and reopening it, I can compose and send a message with no problems.

I'm getting really annoyed with having to save drafts and then reopen every time I want to send a message. Does anyone know why this is happening and what I can do to fix the problem?

Thanks in advance!

EDIT: This also happens when I'm replying to a message. Send icon just isn't there.

The next time it isn't there, right click on the tool bar and choose "Customize", find and drag the send button into the tool bar and maybe that will make it stay there.