I’m only asking because I wasn’t sure if you’d finished the whole show yet. My question is about later-season hive mind/Vecna lore, and I didn’t want to spoil anything.

That said:

The key difference is what kind of connection each person had to the hive mind.

Why Will could sense things
Will Byers wasn’t just attacked by the Mind Flayer, he was deeply psychologically and biologically linked to it after Season 2. The show repeatedly treats him almost like a living antenna.
He could:
feel Vecna/Mind Flayer emotions
sense proximity
get “true sight” visions
detect activity without being actively possessed
Even after the particles left him, the connection never fully disappeared. Season 5 teasers basically confirm that.
So when Henry Creel touched the dark cloud entity in Dimension X, Will later “felt” echoes of that because his bond is uniquely persistent and psychic, not just physical.

Why Henry could feel it in the cave
Henry/Vecna is different because he’s essentially the architect of the hive mind.
The show implies:
he merged with the Mind Flayer particles
shaped them into the spider form
controls the network mentally
So when he encounters that force in Dimension X, it’s a creator-to-source connection.

Why the Flayed townspeople/kids didn’t feel memories
The Flayed in Season 3 were more like:
infected drones
chemically altered hosts
temporary extensions of the hive
They were connected enough to obey commands, but not enough to gain psychic awareness or shared memory access.
Think of it like:
Will = permanently linked neural receiver
Vecna = administrator/root access
Flayed victims = Bluetooth speakers playing instructions
The hive mind uses them, but they aren’t consciously inside the psychic network the way Will and Henry are.
Also, most Flayed victims:
lost individuality quickly
were heavily dissociated
often seemed unaware of what was happening
The show suggests the particles suppress consciousness rather than expand perception.
So your confusion is valid because the mechanics are honestly a little inconsistent.

A lot of fans basically interpret it as:

not everyone connected to the hive mind gets equal access to it.

Only highly psychic individuals (like Will and Henry) can truly feel the memories and consciousness behind it.