Just to confirm and reinforce the advice given by @rong42
.....once you hit submit on your comment, you need to refresh the page to see it. otherwise you dont know if its posted or not.
This is one of the many peculiarities, or deficiencies, of the Labs platform, which have confused and angered community members trying to post there.
It has been flagged up in the Labs-feedback thread, and with luck it shouldn't be too difficult for the developers/administrators of Labs to rectify. But there is a long list of problems to be corrected in the Labs site, so don't hold your breath!
A quick and easy interim step would be to put an explanatory sentence into the "How it Works" page in Labs (or at the top of the Comments text-box) telling people that if they post a comment they must then refresh the page to see it.