How can I fix shower bath mixer tap?

I've got a mixer tap on my bath with shower hose connected. When you want it for the bath you press button lever down and lift it up if you want shower to work. The problem I have is when I lift lever up and turn on shower the lever comes back down. The only way I can use shower is by putting a clothes peg under switch to keep it up and this obviously isn't a permanent solution. Is there a washer that I can replace to keep it in the raised position?