i replaced two shorted xs4202 diodes in my 12v DC power supply, i used it for one month and the new ones i had put also shorted, what might be the cause of this failure?

 

Bad or counterfeit diodes, excessive voltage applied to the diodes, excessive load currents..it is a wide open guess without more information.

It may help to compare the voltage and current stresses the diodes are seeing with the specifications in the datasheet.