Has the client had any previous injuries or orthopedic issues in the shoulder?
There could be a weakness in the non-dominant shoulder. Are the clients hands spaced evenly on the bar when doing a bench press? There could be a few different reasons for the imbalance so it’s hard to tell.
Using dumbbells for chest press as well as doing single arm work/presses presses may help to strengthen each side and correct imbalances.
Imbalance is pretty common since we do different tasks with our dominant and non dominant sides. I’ve especially seen strong non-dominant arm and chest in my clients (and me!) who are moms. We do our detail work like opening doors, writing checks, pushing buttons, etc. with our dominant hand while carrying our kids in our other arm.