The Falcons slipped to a 42 run defeat at home to Leicestershire – their sixth defeat in eight North Group fixtures this season.
And Cork, head coach in the T20 format said it was simply not good enough.
"It was a poor performance and some of the players need to take a good look at themselves and understand how to play T20 cricket.
"We haven't been batting badly all the way through the competition but the last two games really haven't been good enough and I think they know that."
Louis Kimber’s maiden Vitality Blast 50 helped Leicestershire Foxes secure the win at Derby.
He struck 53 off 28 balls as the Foxes recovered from 80 for 5 to post 174 for 8 with Scott Steel scoring 46 and skipper Colin Ackermann 37.
Matt Critchley’s 2 for 23 and 2 for 30 from George Scrimshaw put Derbyshire in a good position until Kimber’s late charge set a challenging chase.
Derbyshire were well placed at 60 for 0 but they collapsed against the Foxes spinners to 132 all out with eight balls to spare.
Callum Parkinson , Ackermann and Steel shared six wickets to seal a second successive win while Derbyshire have now lost six matches.
Leicestershire began slowly against disciplined bowling and then faltered just when Steel was pressing on the accelerator.
He ramped Logan van Beek for six and pulled Finn Hudson-Prentice over the ropes after Josh Inglis had carved to mid off.
Scrimshaw and Critchley gave Derbyshire momentum and halted Leicestershire’s by taking four wickets in four overs with Steel skying a pull to mid on.
Critchley’s second over was a big one as Rishi Patel missed a sweep and Lewis Hill drove to long off.
It left Ackermann and Kimber to rebuild carefully before launching an onslaught that brought 47 from the last three overs.
Kimber drove Conor McKerr for six and although Ackermann failed to clear long on, Kimber twice launched Hudson-Prentice into the stand as 19 came from the final over.
It left Derbyshire a stiffer chase than had looked likely but they began well with Harry Came taking three fours from a Gavin Griffith’s over.
Luis Reece drove and pulled Griffiths for six to take Derbyshire to 55 without loss at the end of the powerplay.
Parkinson made the breakthrough in a maiden over in which Came was bowled and Reece was run out for 19.
Derbyshire needed 104 from 10 overs but the pressure exerted by the spinners told as Billy Godleman was lbw to Steel and Leus du Plooy drove Ackermann to long off.
When Critchley fell in the same Ackermann over, Derbyshire had lost five wickets in six overs to slide to 85 for 5, a position from which they never recovered.