Sorry for late reply.. This one is quite simple.
Look at the Log.
CapMonster cannot recognize that specific image, maybe because it says "Click verify once there are none left." or maybe for another reason.
CapMonster also does not have 100% success rate with ReCaptcha2 images.
Solution
You have 3 options...
1) Send to service like 2Captcha, they do support ReCaptcha2 images and they do support "Click verify once there are none left."
2) Change to ReCaptcha2 Associations -
http://zennolab.com/wiki/en:addons:capmonster:rc-associations
It has a much higher success rate than ReCaptcha2 images.
3) If you really do not want to change from RC2 images, then try putting in a "retry" loop
This is for when CapMonster cannot recognize the images, then it automatically clicks on the Retry icon
So your template will look something like...
Snippet for "Click Retry"
var tab = instance.ActiveTab;
var clickRetry = tab.FindElementByAttribute("div", "id", "recaptcha-reload-button", "regexp", 0);
clickRetry.Click();
Best to use that in a loop with a counter so that it does not get stuck in infinite loop...
That is all I can help you with.