Free Writing Warmer

Students do a free writing non-stop on a given topic for three minutes. When the teacher gives the topic they start the task. If nothing comes to their mind they continue writing the last word where they are stuck. The aim of this warmer is continuity and that students can speak more fluently when they are asked. I've tried it several times and it works, however, some of my students write a pretty long work but the weaker ones sometimes only repeat writing the title.