The possible causes can be divided into 3 groups: above the diaphragm, the diaphragm itself and below the diaphragm.
Causes for above the diaphragm are pulmonary fibrosis or pleural fibrosis. Diaphragmatic causes include eventration and paralysis. Below the diaphragm includes liver pathology (eg liver abscess & HCC) and sub-diaphragmatic fluid collection.
My next recommendation would be to do an ultrasound to assess for liver pathology and diaphragmatic excursion. If the diaphragm does not move well with respiration, then a diaphragmatic paralysis ought to be considered.