Peat based soils may work for rooting cuttings, but for bigger trees you really need to use a much freer draining soil that will fit the needs of the serissa. I have found mine that were rootted in a sandy mix did much better than the same generation rooted in a peat based compost mix.
But the yellowing and lack of vigor is not something I would worry about. Serissas are tempermental and will yellow or drop leaves when moved or watered improperly. The best for you to do is to make sure there are no more pests, and water only when needed.
Just a note, overwatering often causes yellowing and dropping of the leaves and a loss of vigor, so watch out for that
