UKIP leader Nigel Farage launched his party's manifesto on Canvey Island in the Castle Point constituency in Essex.
But Rebecca Harris retained the seat with 23,112 votes - an increase of more than 3,000 votes from 2010.
Mr Carswell held Clacton, but his majority was cut by 9,000 votes compared with last year's by-election.
The Conservative candidate Giles Watling came second in Clacton again, but his vote increased from 8,709 to 16,205.
Mr Carswell, whose defection from the Conservatives sparked the by-election, saw his vote drop from 21,113 to 19,642.
Following his victory, he criticised the first-past-the-post voting system for UKIP's failure to take more seats.
"I hope that those vying to get their hands on the levers of power will have the modesty to remember that there are many people who feel under-represented," he said.
"Here, in our part of Essex, people voted UKIP and they got UKIP, yet across the country about five million people will have either voted for UKIP or for the Green Party.
"That failure to translate those five million votes into seats is less a reflection of how my party or the Green Party campaigned; rather it tells us how dysfunctional our political system is."
In Castle Point, UKIP's Jamie Huntman was second with 14,178 votes, while the Liberal Democrats' Sereena Davey managed just 80 votes.
UKIP was third in its target seat of Thurrock as second-placed Labour narrowly lost to the Conservative Jackie Doyle-Price by just 536 votes.
The Conservatives have also held Basildon & Billericay, Basildon South & Thurrock East, Brentwood & Ongar, Chelmsford, Epping Forest, Harlow, Rayleigh & Wickford, Rochford & Southend East, Southend West.
The Liberal Democrats' Sir Bob Russell saw his 7,000 majority in Colchester wiped out as Will Quince reclaimed the seat for the Tories for the first time since 1997.
Follow the latest from the counts via the BBC's election live service constituency by constituency.
The Conservatives have mostly held off the challenge of UKIP in Essex, but Douglas Carswell has retained Clacton for the anti-European Union party.