It’s easy to “out eat” a workout and there’s shifts that happens in a person’s metabolism based upon what they eat and when they eat it. Also gender, age and body type factors in, but start / troubleshoot with simple things, the basics and go from there.
There are different ways to workout that will boost calories burned, but then you have to consider the client and what type of training they can handle physically.
My attitude is that if the client has not been medically diagnosed with a condition that they can’t lose weight then they CAN, and an answer is there to be found!
Much luck to you!