Two-time Olympian Tonia Couch suffered a surprise defeat in the 10m syncho event at the National Diving Cup.
Couch, 26, was diving with a new partner, 16-year-old Lois Toulson, but the pair finished second in Southend behind Georgia Ward and Emily Boyd.
It leaves a question mark over who will be selected for the final Olympic qualifying event in Brazil next month.
"We've still got a lot to give and with another month's training will be stronger," said Couch.
Teenager Toulson won gold in in the 10m platform event at the European Games in Azerbaijan in June.
Ward - who trains with Boyd alongside Tom Daley at the London Aquatics Centre - told BBC Sport: "It means so much to win and I know how much it means to Emily too. We've both worked really hard."
Boyd was diving in her first national event, having switched nationality from her native Australia in 2014.