I'm not sure how you teach them but there are many resources online that might help you. If you Google, 'teaching creative writing' you'll find lots of websites that provide worksheets, lesson plans, etc. I think creative writing is hard because you need to express thoughts and not information. Perhaps, you are asking your students to write about things they're not comfortable with? If they're just learning, have them write about things they know, like, perhaps, the city they live in or their families.