[open] Losing first keypress in self-paced reading task

Hey,

I have a self-paced reading experiment: participants press a button when they have finished reading the current word and are then shown the next word. For this, I loop over pairs of showing a text canvas and collecting a response; one loop for each sentence.
For some reason, in 90% of cases, the first response to the first item in a loop is ignored, so the first item presented requires two keypresses - but only some of the time!
I have no idea why. Any ideas?

I develop this on a Mac, but have observed the same problem on a Windows computer.

I'm appending the critical part of the script, and there's a pastebin to the full one at the end. I can also upload the full script, if that is required.

    from openexp.keyboard import keyboard
    from openexp.canvas import canvas

    cs = []
    for w in words: # words is a long list ...
        c = canvas(exp)
        c.text(w)
        cs.append(c)

    end = None
    rts = []
    iscorrects = []
    key = None

    judge_keyboard = []
    judge_keyboard = keyboard(exp, timeout=None)
    judge_keyboard.flush()

    blank_canvas.show()
    self.sleep(250)
    for index, c in enumerate(cs):
        start = self.time()
        c.show()
        judge_keyboard = keyboard(exp, timeout=None)
        key, end = judge_keyboard.get_key()
        print(key)
        rts.append(str(end-start))
        if (key == "space" and index != w_i) or (key != "space" and index == w_i):
            iscorrects.append("1")
        else:
            iscorrects.append("0")
        blank_canvas.show()
        self.sleep(100)

    judge_keyboard.flush()

http://pastebin.com/Cp2QB2jd