Gmail seems to be the problem. I have the “Send Again” option on iOS when I select mail in my Sent folder and hit the “reply arrow,” **for mail I sent using my regular email account.** But for messages I originally sent using gmail, the “Send Again” command is simply missing.
I bet it’s because Gmail organizes the folders differently, so that sent mail is in a folder called All Mail. (There’s also a Sent folder, but I think it’s in the wrong place in the hierarchy?)