There are two places to get a clog- in the valve in the top of the can, and in the spray head. If the clog is in the valve, the can is shot. The only hope is to decant it and use it in an airbrush or with a regular brush.
In addition to the "turn it upside down and spray until paint stops" cleaning, you must put the cap back in place. In some new spray cans you do not even turn the can inverted and spray. The cap fits so tightly that is the main protection against drying in valve or spray head. There is an inner cap portion. If that gets damaged you need to find an old cap and use that to protect against drying.