I hope you don't use 資質差 as the "reason" (or excuse, pardon my being frank) and then do not attempt to find out the real cause.
Children that have difficulties with maths (especially essay question) is because they do not understand the question. That is, they don't know how to turn the question into mathematical expression. If you have the patient to explain them step by step, they will get it sooner or later.
For English, most oriental students have problem because of the lack of exposure, and they are lazy to memorize the spelling. English is the easiest foreign language, at least the grammar is not as complicable as Latin-based language. Do not force the child to memorize individual words, but to get him/her to have interest to read more - subjects that they are interested in. When we read more, we will pick up those words eventually. Don't force them to check up dictionary for every word. Tell them to just guess and skip. Only check when they come across 3-4 times and still don't know then they should check it up. Just keep the interest to read the story first.
Language is something you can't grasp without over 20 years of continuous exposure. As long as the interest and need is there, they will learn it. Like me, my English marks were terrible in school, but I keep learning after I started working and here I am. So tell your child, as long as he/she doesn't give up, he/she will be OK.