This seem to stem from a misunderstanding. Lemmy and Mastodon have different goals. Users of each generally don’t want the other’s content generally showing up en masse.
If you want to see something from Mastodon, you can usually get it on Lemmy, and vice versa. I don’t often see Mastodon posts either, but that’s because I only follow Lemmy communities, so I only see them when someone on Mastodon tags a Lemmy community in their post. If you want to see more Mastodon content, see if you can follow some users or hashtags from Lemmy.
But that might not be supported; I haven’t tried. If you prefer Mastodon, I’m pretty sure that you can follow Lemmy communities from there.