why do some of the responses/ comments to the discussions not sent?

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many times what happens is that after responding to a discussion by a way of comment or response, they are not sent though u press "post response", button. the browser shows the message "Done" at the bottom left hand corner and the screen is blank. The no. of responses sent which appears in a bracket also does not increase. i dont understand what to do with these situations, as even if u try sending the response to such discussions again, the same procedure is repeated. can anyone tell me what is the reason for such situations?