Writing in a second language is hard. I teach college-level international students who must take two or three semesters of ESL Comp I and Comp II (plus a remedial course) before they are permitted to go on to more advanced courses such as technical writing. The difficult aspect of teaching these classes is figuring out why the students' English has not progressed beyond a middling level of function.
These are bright students. Many speak multiple languages. With extremely few exceptions, the students are respectful and well-behaved. The obstacle isn't their personal behavior habits.
As the years progress, however, I see that HABIT -- not of inattentiveness, but habit that builds over many years -- is the first obstacle to overcome. By the time a person is college age, the person has hundreds (maybe thousands) of habits regarding language usage. Some of the habits apply both to spoken and written language, but very often, speaking and writing have different habit issues.
My students listen carefully when I instruct them about a grammar issue. Then they write, and they don't implement the correction. Why? Because knowing how a correction applies is never as easy as digesting the correction presented abstractly. The students can't be blamed. They need help in the moment of applying the instruction to the real piece of writing.
So far, there are only two things I have discovered that foster real progress in writing in a second language: (1) at least five hours a week of writing papers, and (2) the professor editing all the grammar errors in the paper and having the student retype the paper, implementing each edit.
Both of these procedures are laborious for the student and the professor. Marking every grammar and word choice error takes so long, no professor can do it more than a few times per semester. Yet it is the one act on the professor's part that students say helps the most. If the students can only get that level of help a few times per semester, how can the students progress?
It is good to know that sheer practice in itself makes a lot of difference. I have had students whose written English at the beginning of the semester made me feel that there was no possibility that they could progress enough to pass the course. But they did. These students gritted their teeth when they got low grades on early papers and rewrote the papers. They tackled every assignment given them (and there are many) without yielding to discouragement. They pulled ahead of students whose English was initially better, but who didn't work as hard. They made progress that I would never have predicted.
Years ago, I read in my childrens' chemistry book that getting water from 211 to 212 degrees took as much energy as getting it from 1 degree to 211 degrees. Breaking past not-quite-boiling to boiling takes immense energy. Applying that to writing in a second language, to break past that middle-level-habit barrier, intense energy has to come from both professor and student. When that happens, so does significant improvement.