This is a difficult problem. It really needs to be addressed by a professional and not in English class, but if you have these students in class, you also need to attend to their needs.
I don't do much writing until students are 4th-6th grade. So, I haven't run across this problem. I do have some younger students working with the alphabet, but not heavily enough that this would present itself or make a dynamic presence in the classroom.
There are some speech pathologist that are forum members, but I'm not sure about reading specialists. Maybe someone else can offer some advice or share some experiences ...
As far as difficulty remembering class to class, I have a few of those students
There are a spectrum of students. Some can remember easily. Some are interested in English and remember because they are interested. Some are interested but aren't very gifted. Some just aren't interested. Some are disinterested and aren't very gifted.
There's not much that we can do about their physical ability, but we can (hopefully) do something about their level of interest. We can encourage them to remember more or try to remember more class to class by providing tasks that require them to remember, tasks that lead to goals they are interested in. For example a sticker for remembering all the words, a short TV show, a game of tag outside, a walk to a nearby park, 15 minutes on
MES Games 
or whatever interests them.
It's not easy, because it's individual. But, you need to find that motivating force and use it to help them remember.