"We played Leicester two times and three of their goals were top-corner worldies," Frank told the BBC.
"I knew Maddison could do it and [Youri] Tielemans earlier this season. Maybe not Castagne, but fair play to him."
The loss sees Brentford remain in 15th on the table, eight points above the drop zone.
Meanwhile, Leicester are into the top-half of the table on 36 points.