"The frustration is so massive," Pochettino told talkSPORT radio, saying a strong first-half performance gave way to a lack of calm after the break.
Defender Marc Cucurella shot straight at Mark Flekken after a lofted pass by playmaker Cole Palmer.
But Brentford soaked up everything Chelsea could throw at them with centre forward Nicolas Jackson, back in the starting 11 after an injury, unable to provide much of a threat.
Brentford's third league win at Stamford Bridge in as many seasons meant that they leap-frogged their neighbours into 10th spot, while Chelsea dropped to 11th ahead of the rest of the weekend's fixtures.
One name linked to the club is Brentford and England centre forward Ivan Toney who scored 20 times in the league last season but who is currently serving an eight-month ban for breaching betting rules.
Persons:
Ethan Pinnock, Bryan Mbeumo, Mauricio Pochettino, Pochettino, Chelsea, Pinnock, Robert Sanchez, Thomas Tuchel, Graham Potter, Noni Madueke, Marc Cucurella, Mark Flekken, Cole Palmer, Nicolas Jackson, Ivan Toney, William Schomberg, Hugh Lawson
Organizations:
Brentford, Chelsea, Arsenal, Blues, Stamford, Thomson
Locations:
London, Argentine, U.S, England