Maybe, maybe not. The lesson Harry learned teaching Molly is his own talent and power isn't set in stone, he can always improve. Oh he figured out long ago that he could improve his "gadgets," his shield bracelet, his power rings etc, but he never figured out he could improve himself. Why? I think because Justin wanted him to learn what he wanted him to learn, thus he could keep control of him. So I think he drilled into Harry's head from a young age that if he wasn't good in a certain area, like veils, he wasn't going to get any better at it so don't waste time. He wanted Harry to know kaboom fire magic, but he never taught him how to control it. So for many years Harry assumed he was only good at blowing up buildings but not the subtleties of fire magic.
I think it's something else: Harry is mostly
reactive. Mostly he reacts to threats & opportunities & other needs-of-the-moment.
In large part this is simply the needs of his life. He's a working PI, so most of his magical workings are in pursuit of solving his cases, and the needs of self-defense as he often works in high-threat situations; in the end, there's only so many hours in the day... and that stuff filled up (almost) all his hours.
Invisibility-style veils were
hard for him. Fire-magic (as a counter-threat) was
easy; self-protection potions (speed-boost, health, "don't-notice-me-I'm-bland", etc) were
easy. So he never put in the long hours he'd have needed to explore/improve veils, because he had solutions that were "good enough."
When Molly became his apprentice, however, he needed to react to
her... and suddenly, "veils" (and other magics he hadn't pursued) moved
way up his priority list.
