So in my very personal opinion it is obvious why students are not being as proficiency as expected, they are not being well assessed. And it is because teachers are not being trained enough. Most of us started to work one or two years ago and we did not know anything about how to evaluate student's performance but our own experience. I bet that most of us did the same that was done to us with our students. Now after all we have read and done, what do you think you did right? What did you do wrong? Is there any way you can repair it? If there is, do it right away because remember that we, as teachers, can even enhance or destroy student's love for whatever they are learning.
Remember that not everything is about written texts and that in English especially we have up to five skills to include on a test. So try to be as reliable as you can on your assessment and make sure you get to know each case, I know it is time consuming and sometimes get on your nerves but I am pretty sure that when you see the results they will be just awesome!