i'm sorry that you are having a problem with your unit. here is the website that you can order a new cap for. if you have any more questions please feel free to ask.
I have the same model; it has a little disk about the size of a dime at the bottom of the well where the mist comes out. That metal disk gathers white scale on it, Clean that off. The best way I have found is with a Waterpik turned all the way up. The scale keeps the disk from vibrating.