Porosity can be caused by several factors.
A simple breeze can blow away the shielding gas.
Too little gas flow which does not provide enough shielding of the atmosphere from the molten weld puddle.
Too much gas flow which results in a venturi effect that will actually draw atmosphere in and contaminate the weld pool.
If the base material is not clean, the contaminants will boil out in the weld puddle.
If the filler materials are not clean or have absorbed moisture those can cause porosity.
If the base material is colder than the dew point, moisture can form on the surface.
In correct technique maintaining too long of an arc can let atmosphere be drawn in.
Welding in a corner can cause a ventri effect.
Excessive voltage can sometimes cause porosity.
Removing the Tig torch too quickly from the weld puddle after extinguishing the arc or too short of a post flow setting can cause porosity.
I'm sure others here can chime in with other factors that escapes my mind at the moment that can cause porosity, but without more information about the welding situation, it is difficult to come to any conclusion or offer any advise.