Hello! Based on my experience, I advise improving their visual skills, because they are closely connected with their spelling. Tell them to practise memorising where things are at home first, then tansfer to the clasroom. If you have some visual material on the wall, ask them to try to remember what that is when they close their eyes. Then start focusing on spelling , but now use the text. Another important factor in dictation is listening. Practise it with them. I do believe that those two combined together will give results. Best wishes!

There is an excellent book on Dictation by Mario Rinvolucri and Paul Davis published by Cambridge University Press. I'm not sure if it is still in print but you may be able to obtain a used copy. This includes ideas for grading dictations to make them more or less difficult.

Although dictation has very little real-life value, it is a very useful learning procedure providing ear-training, spelling practice and intensive comprehension.