My thermostatic shower seems to stop if I do.not use frequently. sO have to leave it for a long time before the water comes out.Could this be an air lock problem?
try www.showerdoc.com. You'll obviously need the make and model of shower. Once you have that you shoud be able to go to the repair section of the site (youll need to look around fo rit) and you'll see a load of questions about various showers and often advice about how to fix.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.