"We just have to say that we haven't been good enough yet," Sexton said in an interview with Irish television channel Virgin Media One broadcast on Wednesday.
"(We're) unbelievably frustrated. How can you not be when you're doing as good a preparation as you did last year, you're training as well as you did last year? You're doing everything right and things aren't just clicking. No one cares more than us."
Sexton said last year's clean-sweep was achieved with "a little bit of luck along the way" that he felt had been missing this year.
"Probably didn't get the bounce of those balls against England and now we're trying to find that sort of performance that we need to revive us again and get us going," he said.
Ireland host France in the next round on March 10 before finishing the campaign with a trip to currently unbeaten Wales on March 16.
(Reporting by Rohith Nair in Bengaluru; editing by John Stonestreet)
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