Open ground

In a near complete kitchen remodel I have two outlets that are showing open grounds, using a "snapit" circuit tester. My house was built in the 50's and wired with metal boxes grounded through metal flexible conduit. The remodel work then added plastic boxes with romex wiring. An electrician sent out by the contractor said the open grounds were not a problem since they are both on a circuit with a GFI plug. The GFI outlet tests correctly and the ground wires in the two down-stream outlets are secured to the green screw on the outlets so I'm not sure what the problem is. Should I ignore the problem as the electrician suggests?