OK, I think I have this straightened out. The situation was that only my first couple of PixelFed posts showed up in my Mastodon timeline, anything more recent did not. Somewhere along the way, I unfollowed and refollowed my Pixelfed account, to no effect. Today, I went into my Mastodon settings and saw that my Pixelfed follow was “Pending”. I canceled it and followed my Pixelfed account again, and all is well.