Age is not a factor. It is with the subsitutes. But both Goex and Swiss are real black powder.
Your experience is contrary to what really happens when Goex and Swiss are tested side by side. Swiss is a higher grade powder and will give 10% to 25% more 'oomph' for the same charge than Goex. Of course both must be same grade. e.g. for pistol use 3Fg.
But, go with what you like. Use up what you have on hand it won't go bad.