Mixets can be tricky. Mixet of course is out of business, so all the parts are aftermarket, of varying quality. There are some thin flat washers that need to be used . They act as spacers and friction washers. Google around the net to try and find a parts breakdown showing how the go together.
Mixets worked ok when they were new, but when they start giving problems, there are some internal wear problems on the valve body, which cannot be fixed with just new cartridges. At some point, consider replacing the whole valve with a new moen or delta