Mark compares the Spirit descending to a dove. Luke says that the Spirit descended in a bodily form like a dove. Imagine for a moment a dove descending onto Jesus. What would that have been like? Would it have been a quiet, soft glide of the bird as it gently approached Jesus? Would it have been more noticeable, like a dive bombing bird after french fries? Either way, I imagine those around paid attention. It was not an ordinary action. The voice from heaven probably added some punch to the message, too. Could anyone have gone away that day doubting that something extraordinary had happened?
Then the Lord called, ‘Samuel! Samuel!’ and he said, ‘Here I am!’ and ran to Eli, and said, ‘Here I am, for you called me.’ But he said, ‘I did not call; lie down again.’ So he went and lay down. (1 Samuel 3:4-5)
In this passage, we see that Samuel is hearing the Lord, but his mentor, Eli, doesn't recognize who is calling to Samuel (at least at first). It takes three times for Eli, the Priest, to clue into what is happening.
In both incidents, God isn't being subtle. It's the listeners who need to listen and see.
The beginning of chapter 3 in 1 Samuel says, "The word of the Lord was rare in those days; visions were not widespread." How often do we say something similar? How often do we doubt that God is communicating with us? Maybe God is, and we just aren't listening.