I would check your argon gas, make sure it is up to standards Mil-A-18455 calls for 99.985% purity, the regular argon from my supplier is 99.997 purity and I use a ultra high pure argon for titanium and aluminum work that is done to AWS D17.1 spec at 99.999% purity.

If your torch is old and has the rubber lines I would replace it with new vinyl lines. Also replace the regulator to machine argon line with vinyl as well as any purge lines. The older rubber lines will hold moisture and other nasty contaminants that can create porosity.

Make sure you are doing good cleaning of all your parts with acetone, stainless steel wire brush and a final acetone with clean lint free clothes. Clean your filler wire with acetone. I wear a white cotton glove for holding the filler wire to keep hand oils off it.

Regards
Jim