If limited vocabulary is holding back the level of writing of students in your class then you need to do some specific teaching in order to raise their vocabulary repertoire. e-Learning has much to offer here in scaffolding this drawing out and harvesting of rich vocab.
A great place to start is by searching for video footage on a topic of interest. Youtube is a great source for these. Check out this video of the bald eagle catching a salmon. Play it through several times stopping at appropriate parts to discuss features such as the wings, beak, claws/talons, eyes and how these help the eagle when hunting for food. Build up vocabulary as you do this.
Now have your students do a Google search for the topic about which they are writing e.g. a bald eagle catching its prey.
For example see http://www.baldeagleinfo.com/
Use the wonderful graphics and use of part/whole thinking to look at the features of the bird that will come into play as it swoops to catch its prey.
What are words that describe the look and use of the wings, beak, eyes, claws? Make a list to use in your writing. (Now you have drawn out what they know through observation - but you have added nothing new.)
Now access the information on the page. For struggling readers download "Natural Readers" free text read software. Set up the miniboard on your desktop, and now whenever they highlight text it will be read to them. They can now read along with the reader and harvest words they would like to use in their own story. Note it must be text that they highlight to read not pictures with text included.
This will bring in words like foveae, attacking, talons, screeching, soaring, gliding, snatching that will add real impact to their story of the eagle catching its prey.
If students then need a way of compiling the new vocabulary and sorting it ready for writing, using a tool like Popplet Lite app or popplet.com web 2.0 tool allows them to brainstorm and then sequence the popples prior to writing. You can check this out further in the "Organising your ideas" section.