Sometimes a piece you want to win is perfectly well defended, so capturing it directly just loses material back. The fix is to deal with the defender first: capture it, trade it off, or chase it away, and the piece it was protecting becomes free.
This is one of the most common ways tactics actually work in real games. Before giving up on "that piece is defended," ask what's doing the defending, and whether that piece itself can be dealt with.
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Black's rook on d8 looks safe. The bishop on b6 is the only thing that isn't defending it directly, but it's in the way of the real plan.
Which move wins material, with more to come?
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