Open Ground

While testing my outlets with a circuit tester, two of them indicated an "open ground" condition; the other outlets registered normal. The circuit is 20 amps with four duplex receptacles, one combination switch outlet, and one dimmer switch.
I opened every up every single box and checked for proper grounding connections(to both box and receptacles, pigtailed)and every one of them were properly wired.
I guess somewhere in the house is a ground wire that is not connected that these two particular outlets were tapped off from?
Had a similar problem with another circuit; however I managed to find the culprit-an unconnected ground wire in a wall switch.
Is the "open ground" a big risk?
Unfortunately, the basement is finished, so short of tearing apart the ceiling, not really sure how much more I can do to try to find this problem-already spent days to no avail!
Thanks for any suggestions, advice, and alternative solutions.