PTC page not giving credit

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I am new to the PTC game, but I have found two sites that time you at the top of the page you click on and then give you a green check or something like that to show you have received credit. They also say that if you get a red check or a red X, then you have another PTC page open so you won't get credit. The other day I was doing both of these sites at separate times and got red Xes when no other page was open. Not for every page, but for some of them. Does this ever happen to anyone else? Am I doing something wrong?