How to fix an "open ground" on a receptacle that seems grounded?
Well, you either have: 


No connection at the box despite appearing to have one (quite
possible with aluminum wiring if the appropriate methods are not
followed, from what I understand - but I have limited personal experience there)


or 


the ground wire to those boxes is broken or not connected somewhere
between those boxes and the main panel. Which actually makes lots of sense if you have two boxes in this state; probably one is connected to the other, and the wire is broken or not connected to the one that the other connects to.


I trust that as an aluminum wiring house owner your new receptacles are marked as suitable for aluminum wire?

Edit: No? Seriously?

As @Speedy Petey says, that really isn't an acceptable alternative to using the correct receptacles. House fires are not much fun. As for tracking down the problem, if this end is connected properly, you need to check the far end of the wire for a proper connection - you may find that you can identify which outlet is "next in line" by identifying the power wires feeding the probelm outlets - look there for a poor connection between the two ground wires. 

@Chief Two Pencils seems to be failing to observe that you have aluminum romex with a ground wire - the problem is not a lack of ground wire, it's a lack of that ground wire being connected at some point.

You are failing to understand that the connection to the box does not "magically ground the outlet" - the box is being grounded (or rather, should be) by being connected to the ground wire coming from the cable. That wire is not connected all the way to the main panel ground from the two problem outlets. You, or a competent electrician (I'd really have doubts about continuing to use one that suggest that anti-ox paste is an adequate substitute for using CO/ALR or COALR marked devices) need to track down where that lack of connection is happening, and fix it, with a connector approved for use on aluminum wire (and it may well not be connected now becasue it was not connected with a suitable connector, or not properly prepared.) 

The US CPSC advocates a more through approach of pigtailing copper wire to the ends of the aluminum wire with an approved connector to eliminate the aluminum connection fire hazard in a permanent fashion. Then you can use nice normal Cu-only connectors.

This is not a place for "creative" solutions. The house you burn down may be your own.