Emma,
This happens to me too. Check the washer, by carefully pulling the drawer out a little bit when its filling, to see if there is a trickle of water into the fabric softener compartment You may have to wait for thr rinse cycle). If there is, pull the drawer out and there should be an inverted round cylindrical cover over another tube in the softener compartment. If you pull the cover off the inner tube is likely to be blocked. Cleaning and reassembley should cure. If you are able to remove the drawer completely, you can offer a trickle of water into the compartment over your sink. What should happen is the compartment should start to fill. When it's almost completely full a syphon effect is created and the water should drain through the centre tube. Turn off the tap and the water should continue to flow till the compartment is empty.
Hope this helps,
Dave B
July 2010