Jazz Carlin and Fran Halsall missed out on automatic Olympic qualification despite winning their finals at the British trials in Glasgow.
Carlin, 25, won the 800m freestyle and Halsall the 50m freestyle but failed to meet the qualification times and must seek Rio 2016 places in other events.
"It's just not good enough," Halsall, 26, told BBC Sport.
Max Litchfield, 21, won the 400m individual medley to become only the fourth GB swimmer this week to qualify.
He beat favourite Roberto Pavoni, who had won the past five British 400m individual medley titles.
Litchfield knocked more than five seconds off his personal best to triumph in four minutes 12.05 seconds, three hundredths of a second inside the qualifying time.
"It's a boyhood dream and I knew I had it in me, but for it to become a reality is a bit surreal really," he told BBC Sport.
Duncan Scott produced arguably the shock of the night to beat British record holder Ben Proud in the 100m freestyle final in a time of 48.66 secs.
Although 18-year-old Scott was outside the qualification time of 48.16 secs, he is likely to be named as one of the discretionary picks.
"I'm absolutely delighted with that swim and a first British title," the Scot told BBC Radio 5 live.
Aimee Willmott atoned for a disappointing second-place finish in the 400m individual medley on Tuesday with victory in the 200m butterfly final in 2:08.82.
Alys Thomas was second and 15-year-old Emily Large an impressive third.
Carlin, who won in 8:27.49 but needed 8:22.93 to qualify, missed out on selection for London 2012 after suffering from glandular fever in the build-up to the trials.
"It's been a really tough week and, missing out four years ago, I put so much pressure on myself because I don't want to miss this Olympics," she told BBC Sport.
"I know I'm a lot faster than the times I've been swimming here and hopefully I can turn things around later in the week."
Fellow Commonwealth champion Halsall won in 24.48, just 0.09 secs outside the qualification time.
She said a lack of British talent in the 50m meant it was "me against myself as a time-trial".
She added: "I've been training really well and it's the first time in ages I've been completely injury-free, so to swim like that is really quite disappointing."
Halsall, Carlin and Proud follow Chris Walker-Hebborn in missing out on automatic Rio qualification.
If they do not achieve the standard through other events, or via selection for relay events, they must rely on discretionary picks by the selectors.
The team for Rio will be confirmed on 21 April.