We've built and demoed apps that do just this, monitoring certain hashtags and mentions. It does work.

Most likely it's limiting the query of returned results to a certain amount. Maybe 20, maybe 50, etc. This needs increased, etc.

Tell Dork or Shakespeare or whatever agent you're using to "implement infinite scrolling to load more content at the end of the feed". The AamI doesn't seem to always do that and in every app that I've made I have to tell it to do infinite scrolling and lazy loading and pagination.

You can also try changing to relays that aggregate content such as Primal or Nostr.band.