A $15.3 billion ETF—BlackRock’s iShares MSCI USA Momentum Factor—suffered losses due to the timing of a rebalance. This according to a recent article in Bloomberg.

The BlackRock ETF reportedly “went all in on the reflation trade back in May. It has barely edged higher since then as the cheap, cyclically sensitive shares it piled into struggled.” The fund’s quant-based strategy, the article explains, tracks the market’s top performers of the past six and 12 months, and in May upped its allocation to financials and cut exposure to tech companies.

The problem, the article notes, is that the fund’s semiannual rebalance didn’t come soon enough and created a “double blow”—it ended up holding tech stocks longer than its peers when they were lagging, then switched to value shares “at their peak.”

The losses, the article notes, are a “measure of the fickle markets this year that have left Wall Street divided over whether the reopening trade has run its course.” It adds that it is also a reflection of “one of the key questions at the heart of index investing—when and how should a fund rebalance? Too often, and costs ramp up. Too seldom and it fails to keep pace with the market.”

The fund’s “scheduling headaches,” the article concludes, are “far from unique” adding that in the $1.4 trillion smart-beta universe, almost 20% of funds rebalance semiannually (according to Bloomberg Intelligence) and about one in 10 adjusts annually.