Hard to say -- it's been a long time. They just didn't engage me as much. I recently reread one of his later ones, The Songs of Distant Earth, which I remembered liking when I first read it decades ago, but this time I found it rather superficial and plotless, mainly an exercise in worldbuilding.
This happens. I recently had occasion to reread a book I loved when I was twelve:
Cold War in a Country Garden by Lindsay Gutteridge, a sci-fi adventure novel about miniaturised British secret agents trying to survive in, well, an English country garden. Lots of battles against giant insects and Russian spies, as well as plenty of
Swiss Family Robinson-type ingenuity and engineering.
Like I said, I remembered it fondly, but rereading it as a adult I was frustrated by just how flat, stoic, and unemotional the characters were. Lots of competence porn and stiff-upper-lip British reserve; even when a fellow agent gets eaten by an ant or whatever, nobody gets too distraught about it. "Damn shame what happened to Jenkins. Anyway . . . . "
Unlike
The Incredible Shrinking Man by Matheson, there's no angst, no existential dread, no drama or heated emotion. Just boy's adventure stuff, peopled by smart, resourceful heroes who barely seemed to have any interior life at all, let alone libidos. Heck, even when (surprise!) they encounter a couple of miniaturised women, nobody seems too excited by this. It's kinda implied that, sure, the hero and the heroine are going to pair off eventually, but this is mentioned rather matter-of-factly in just a few lines near the end of the book. Before that, there's not a hint of sexual tension or jealousy. Or much in the way of characterization, period.
So, yeah, my older self was less impressed than I was back in 1972 . . . .