Each congressperson's staff gathers all of the comments they get, counts them up, and reports back to the congressperson, passing along anything they think would be useful information; if you write a clear and careful letter about an obscure topic there's a decent chance the congressperson themself will see it. Congresspeople typically have local ties in their area and caring what their constituents think is their job.
It's a serious problem that there are some congresspeople who don't do any local events, send all comments straight to the trash, etc. You should vote those folks out.
In the longer term, we should push for significantly increasing the size of the US House of Representatives to 5–10x the current size and implement serious campaign finance reforms. In combination, these will help make congresspeople more responsive to constituents and less reliant on donors.
A 10x increase would just be chaos.
Not an American, but I've wondered about increasing numbers myself. Certainly, giving each representative fewer citizens to represent could help.
I worry about the size of the bodies, however. Too big, and they become less wieldy. Maybe I'm wrong, but I wonder about other solutions. I was thinking of, for example, 10x the number, but each grouping of 10 has a representative, and they each give proxies on votes. Maybe best though of as, junior representatives. It'd allow more direct interaction, and in a sense you'd be electing regional representative staff for each congressperson.
I guess there are a lot of ways to handle this, but regardless I overall 100% agree.