Diagnostic first then regas is the only way

Several resons  why you cars not getting cold 

No gas that is indicating you have a leak 

Pressure test required 

Eletrical fault

Diagnostic scan required

Machanical fault

inspection required

if on fault foud only then do i regas the car 

Diagnostic skill are very important to do on ant automotive ac system i do not just regas and run .IT IS A LEGAL REQUIREMENT IN THE UK NOT TO FILL A CARS AC SYSTEM AND NOT DO FULL DIAGNOSTICS