mine does this aswell i find it is to do with the washing machine needing a cleanout and also what fabric softner you use i always find mine clogs up when i use comfort concentrate.
to clean the machine the best cleaner is either
http://www.lakelandlimited.co.uk/product.aspx/ !20348
or fresh buster cant remember the name but has a bee on the front its a well known brand found in supermarkets.
also pull the drawer out and clean and powder than has leaked out if it has any there