Ben Ainslie was left "frustrated" in his quest for a fourth Olympic gold as Britain's sailors suffered a mixed day.
Ainslie scored a sixth and a 12th to slip to third in the Finn class, 11 points behind leader Jonas Hogh-Christensen after four races.
In the Star class, defending champions Iain Percy and Andrew Simpson climbed to first from fifth overnight.
But Paul Goodison, the Olympic Laser champion, struggled on his first day of competition, ending 17th overall.
GB 49er duo Stevie Morrison and Ben Rhodes began their campaign with two 12th places to sit 12th, while Laser Radial sailor Alison Young was ninth after a seventh and 10th in her first two races.
Britain's match-racing trio of sisters Lucy and Kate Macgregor and Annie Lush beat Portugal but lost to Australia to give them two wins and two losses in their four round-robin matches so far for a share of fifth.
Ainslie let slip a golden chance to overhaul Hogh-Christensen in race two on Monday after the Dane, who won both races on day one, hit the boat marking the start line and had to do a penalty turn.
But Ainslie, who trailed the leader by six points after three races, was unable to put himself in the best wind to press home the advantage and finished 12th as Hogh-Christensen fought back to seventh.
The Briton ended the day third overall on 22 points, with France's Jonathan Lobert second on 19 points.
"I didn't have a very good day, so disappointed with that," Ainslie told BBC Sport. "It's pretty frustrating, especially the second race.
"I certainly need to pick up my game for the rest of the week. The boat is fine, it's the person sailing it. They are very difficult conditions but, for one reason or another, I really struggled to pick the breeze and pick the right way up the course, which was frustrating."
Ainslie has fought back from worse positions to win Olympic gold medals - he was disqualified in the second race in Athens and was given 26th.
"Sometimes events come to you and sometimes you have to go out and fight for it," he said. "This one is not coming to me, so I guess I'm going to have to fight for it."
Percy and Simpson scored a third and second on day two to lead the Star class. The Britons are tied with Norway on 18 points, with Sweden one point adrift.
Brazilians Robert Scheidt and Bruno Prada, runners-up in Beijing, are two points back overall after a ninth and sixth in races three and four.
"It's getting there, we're starting to rock," said Simpson. "We managed to get past them [Brazil] and sail away in both races so that was good news.
"We've just got to keep sailing as well as we can and not worry about them. There are other teams that are very strong as well. We've just got to keep chipping away."
It was a disappointing opening day to his Laser campaign for 34-year-old Goodison, who scored a 10th and a 23rd to trail Australia's five-time world champion Tom Slingsby by 30 points.
Slingbsy, who went to Beijing as the leading contender but finished 22nd, scored a second and a first to surge into an eight-point lead over Guatemala's Juan Ignacio Maegli Aguero and Croatia's Tonci Stipanovic.
"It wasn't an ideal start, but there are still another eight races to go, so we've just got to look forward," said Goodison, who was forced to do a penalty turn, apparently for hitting a mark in race two.
Morrison and Rhodes were looking to put the agony of Beijing behind them after going to China as leading contenders and finishing in ninth, a result that hit them hard.
The Exmouth duo struggled in the shifty conditions on the inshore "Nothe" course nearest to Weymouth but refused to let it rattle them.
"It's not how you want to start the regatta, but it's not time to panic," said Morrison. "It's about average points, being patient, being tough and dealing with it. We are not out of it."