The Reds shipped three poor goals at the Community Stadium as Brentford ran out deserved winners.
In a desperate first half, Ibrahima Konate scored an own goal at a corner and, having escaped conceding two further goals at set-pieces because of offside decisions on video review, Liverpool fell two behind when a mistake from Harvey Elliott allowed Mathias Jensen to set up Yoane Wissa.
Reds boss Jurgen Klopp made a triple substitution at half-time and Alex Oxlade-Chamberlain reduced the deficit but another error late on, this time from Konate, let Bryan Mbeumo in to seal the win.
The Reds, who would have gone within a point of fourth with a win, have now dropped 23 points this season – one more than in the entire campaign last season.
“With the fans sitting at home or on the bus back up to Liverpool, it’s not good enough for them,” defender Robertson said.
“We’ve spoken about consistency and things like that but we’re not getting it.
