The washer and jumper (as its called - the bit that holds the washer) is getting stuck in your tap, probably due to lime scale build up. If you can turn the water off and unscrew the tap insert from the tap unit itself you can dismantle the thing and polish up the stem with wire wool, and then lubricate all brass bits with vaseline - as well as replacing the washer. OR you can buy a complete new insert if your local B&Q has them in - there are about half a dozen different types though so its best to take the old insert with you.
John
January 2008