The thing that people forget in both the free walk, and the stretching trot circles, is that you're not just throwing your reins away and having your horse dump it's head on the ground trying to eat the grass ;)
The activity of the working gait MUST be maintained.
In walk, the walk must be active, marching, back swinging, and the horse actively taking the riders hand forwards. Where marks are often lost in this movement, is the transitions from medium walk - free walk - medium walk. The free walk itself might be enough for an 8, but if the transitions are below par you'll be very lucky to scrape a 6.
You see a lot of riders simply throwing their reins away from the medium walk and hoping the horse drops its head - then wiggling the reins a bit when the head comes up. The transition to free walk should be smooth, check the test directives - in the Australian tests they ask for contact to be maintained. Keep your leg on, ride towards your hand as you would if asking for a collected pace, and gradually allow your reins.
The transitions to medium walk from free walk is much the same. The most common problem is a rider will have a great free walk, then panic coming back onto the track and shorten the reins abruptly - horse flings its head up, lose the hind legs, back stiffens and your mark has plummeted from an 8 to a 6.
This is a transition that you need to practice, and the method depends on the horse. Some horses will respond best to gradually take up both reins at the same time. Some are best if you take the inside rein up first, some the outside rein. Play around with it until you are consistently having the horse brings it's neck up and shorten the frame back to medium walk, without resistance.
Similar for the stretching trot circle.
Riders throw their reins away, horse puts nose on the ground, hind legs are gone, contact is gone. Then panic and pull the horse's head up back to working trot.
The stretching trot circle is quite challenging for most, if you ride is correctly. You want to maintain the activity and engagement in the hind quarters, keep the shoulders light, and allow the horse to reach forward and downward, stretching from the wither. The contact MUST be maintained.