AppleScript sending Mail messages before file attachments are attached in El Capitan
One workaround is adding a delay prior to send — in my case a delay 1 works but .5 would not, but found elsewhere people that were able to do .2, so experiment to see what works for you:

tell application "Mail"
    set theNewMsg to make new outgoing message with properties {subject:theReportSubject, visible:true}
    tell theNewMsg
        make new to recipient at end of to recipients with properties {address:theAddress}
        make new attachment at after the last paragraph with properties {file name:theTempFileName}
        delay 1
        send
    end tell
end tell