The Bees went down 3-0 at Anfield, leaving them in 14th on the Premier League table.
"We are very aware of it and if you want to be a good team in the Premier League you have to do all the phases well," Frank said.
"But if you are a newly-promoted Premier League team you need to be very good at defending set-pieces and you can't concede soft goals.
"I felt in the first 10-12 games of the season we were brilliant at them, and then for whatever reason we have from conceded set-pieces in recent weeks and we need to do better."
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