Ray Dalio’s Bridgewater Associates spent weeks earlier this year tweaking its investment models to account for unprecedented government stimulus and the worsening pandemic. That hasn’t helped performance. The flagship Pure Alpha II fund has lost 18.6% through Thursday, according to a person familiar with the matter. That’s little changed from the decline it reported through the end of August. This year’s loss in Dalio’s main fund is shaping up to be its worst ever, putting him far behind other macro managers who have posted strong gains in 2020.