My shower mixer valve isn't working correctly. When you turn the shower on, no water water comes out. But if you turn the temperature up to maximum, the shower works but is too hot to use. The strange thing is only this only happens sometimes. Sometimes the shower works perfectly. All the other taps work fine in the house and the pipes running to the shower both have water running through them. The mixer valve is brand new as I replaced the old one because of a similar problem. That mixer valve wouldn't go hotter than luke warm, no matter how far you turned the temperature up. Any advice would be great.