I have read that the dragon dust can cause impaction but I think this risk can easily be prevented by not going overboard on how much of the dust put on greens and insects. I know the bottle does say to be liberal with the stuff but I think no more than a very light coating is needed. Mist the greens after applying as well, to ensure plenty of moisture is injested with the dust.
As your dragon is not pooping and eating normally, I would look into trying other usual remedies for constipation. Give your dragon a luke warm bath for about 10-20 minutes, very light gentle massage on the stomach moving from the stomach down towards the vent (don't squeeze or push down hard, your finger should barely depress the skin). Removing any loose substrate from the cage (ie sand, calci-sand, crushed walnut etc) and using paper towel, newsprint etc will also help prevent accidently impactions. Keeping greens well chopped and insects less than the space between the dragon's eyes, will all help prevent impaction.
You can also feed your dragon a teaspoon of pureed carrots, sweet potatoe or summer squash with his normal meal. The high water and fibre content of the puree often helps move things around. A bit of minerial or vegetable oil can help as well. I would try a combination of baths (maybe 2-3 a day, spaced out as possible) adding the puree to their diet and the gentle massaging for a few days.
If your dragon continues to go without pooping after a couple days of soaking etc, as suggested above, I suggest taking him to a vet, especially if he stops eating or shows any signs of stress or discomfort.
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PHLdyPayne