Gasket can fail due to many reasons other than material of gasket. One of the main reasons could be crusing of gasket , to avoid crushing of gasket ensure that initial bolt loading does not exceed the gasket seating stress y. Gasket may also fail due to improper design of flanges.
What all types of gasket did you use?
Gentle reminder : Please post your queries in appropriate forum. Your post is not realted to Heat Transfer and Thermodynamics engineering.