Hi,
The first thing to remember, Google rankings do not use MOZ DA or PA to decide where a website should be ranked in the results.
The best way to use DA and PA are as an arbitrary measurement that allows you to compare against other sites, so you can see how you are doing against your competitors.
Now without knowing the URLs involved, I cannot check the websites to give any real insights. However, there is any number of reasons why the other sites may be ranking higher than your clients.
They may have better content for the keyword. They may have more backlinks pointing to the domain, they may be answering questions regarding that keyword in more detail.
Without actually being able to compare the sites it is hard to say.
I would analyse the sites that are higher than yours, check the backlink profiles, compare the number of pages and compare the quality of the content then you should have a plan to move forward and improve your rankings.
I hope this helps,
Steve