Glenn Irwin finished fifth in the British Supersport feature race at Donington Park on Sunday.
The Carrickfergus man won Saturday's sprint event and was leading on Sunday but he slipped back in a race won by Luke Mossey with Graeme Gowland second.
Fellow Carrick rider Alastair Seeley, who crashed out with five laps to go, lies third in the series with Irwin now just 10 points back in fourth.
Meanwhile, Jordanstown's Andy Reid won the Superstock 600 race.
The victory leaves Reid, who has won the last two races, three points behind championship leader Kyle Ryde.
Supersport series leader Billy McConnell came in third in the feature race and he holds a one-point advantage over Gowland.
Irwin was the front-runner for three laps before Mossey moved ahead and he held on to win by over three seconds.