Can you try it on another power source with known good voltage? The ATM can be powered from other sources as well, like a car cig/accessory port. These modules can ingest up to 30v, and even 50 in some cases. If the power supply is soggy and won't push past a current limit, this in turn will limit the voltage output.
I ship 24v 6a PSU's with my prebuilts which is usually more than sufficient. A weak 12v PSU might not buck through at a full 12v under load. There is always some percentage of loss in these converters.
I do have parts to assemble them in stock, just haven't done a batch lately. Maybe I'll knock a few of those out and get them listed for those who want some