Cat converter problem please help

i have a 99 z24 with 140,00kms. in december the check engine light came on i got it scanned and the code was P0420-Catalyst efficiency below threshold (bank1) so i replaced the cat with a high flow obd2 approved magnaflow cat and both the front and rear o2 sensors. i drove it for 2 weeks than the check light came back on with the same codes!(P0420) anyone know whats going on? what i can do? please help.:frown: