I know what you mean about running out of things to say. Sometimes you need to change things just for the sake of your own sanity!
Try something like the following;
'This is a good/great/excellent piece of writing that communicates your message effectively. I particulalry liked your use of grammar/1st conditional/vocabulary... You could improve your writing by focussing on vocabulary/spelling/register... I look forward to reading much more from you in the future.'
I can play around with this and tailor it to meet the needs of the student and respond specifically to what they have written.
I hope this helps.