I find that even when I do hold the pick firmly, some picking techniques tend to be quieter than others. Edge picking can be a factor as well, and I find for any picking technique, the closer I am to “no edge picking” (i.e. pick attack is nearly completely flat against the string), the louder and brighter the attack.
What can be frustrating is that with around 30 degrees of edge picking, some techniques are still plenty loud, and others are soft/muted. I think this has to do with how much the hand and arm are “enforcing” the depth of the pickstroke versus letting the string force the hand to rise away from the strings slightly as the pick crosses the string.
My brightest/loudest technique is an elbow-driven DSX motion with the base of the thumb pressed against the guitar face and nearly zero edge picking. The pressure of between the guitar face and the base of the thumb ensures there’s a “buffer” reservoir of downward pressure that prevents the resistance of the string from moving the pick upward.
But it can sometimes just a be a finicky matter of dialing in just the right pick angle/orientation/grip for a technique. The recent pickslanting primer updates have some good material about this in the sections on reverse-dart-thrower wrist motion.
Another thing for me is that attack can seem adequate on an unplugged electric, then be revealed to be too weak once you plug in. So as I experiment with techniques, I make a point of “checking in” on my plugged-in sound more often now. Even that can be done reasonably quietly with headphones (though obviously if your circumstances mean even the acoustic noise can be too much, e.g. napping baby in the same room, that won’t help).