dual minotor setup problem, help!

I have a acer r3700(windows 7) with one hdmi and vga interface. Try to set up dual monitors by using acer v193w which only have vga interface. Monitor a 's vga is connected with R3700 vga by vga cable. Monitor B's vga is connected to r3700 hdmi by hdmi to vga converter.

(1)on " screen resolution" , select"1280x720", select"duplicate those displays", A and B work fine.
(2)on " screen resolution" , select"1280x720", select"extend those displays", A  works fine. B shoes error message "input not supported"
(3)on " screen resolution" , select resolution highter than "1280x720", select"duplicate those displays", A  works fine. B shoes error message "input not supported".

I disconnected minitor b from r3700. connected r3700' hdmi to hdmi interface of HDTV by hdmi cable. A and HDTV work fine.

what causes the error message?