I would suggest a couple of things:
- with a larger group it can be more difficult to feel part of it - there's lots going on
- we haven't yet got to the stage whether the party members really know each other and have bonds to each other in-game. For a homebrew game I'm in run by zatatmando, we had a whole thread called "Character connections" before the game began where all the players jumped in to create shared back-story between the characters. Not everyone knows everyone, but everyone is linked to everyone through that backstory. I think this worked really well in setting up a game and getting player buy-in.
- I think you could probably do more to include "hooks" or "prompts" in your DM posts to drive the narrative forward, for example the below post doesn't have much in the way of prompting for the players to respond to (other than entering, which doesn't advance the narrative particularly)
RoAries says:
The door at the second level opens up, and a human man in armour and an elven man in red-purple robes are standing behind the door with troubled expressions.
"What is it you want?" Says the gruff man.
"You have visitors, Garold sir." Answered the guard.
"Oh, my bad, I'll see to them now. Let the rest of them come in."
The guard returns outside to signal to the rest of the adventurers to come in.
Players need agency, but they don't
always need agency. In the above instance you could have moved the players into the room without compromising their agency significantly.
None of the above is meant as criticism, and all is my opinion only. I think you're doing a fine job of DMing. Please don't be discouraged.
