Volstad has gone 23 straight starts without a "W," after losing on Tuesday to the Astros.
"We can't disregard his last two starts before [Tuesday]," Cubs manager Dale Sveum said before Wednesday's finale with the Astros. "There's still something there that we have to get figured out that we have to get on a consistent basis.
"I think [on Tuesday], his slider was getting hit as good as anything," Sveum said. "His sinker wasn't as consistent as it has been and he threw some flat ones that got hit, but at times he still had his sinker. It's just a matter of being consistent with that sinker and trying to get hitters to hit the top of the baseball."
Volstad had thrown eight shutout innings for Triple-A Iowa on July 27, and he posted quality starts in his first two outings back with the Cubs against the Dodgers on Aug. 4, and against the Reds last Thursday.
On Tuesday, Volstad went five innings, allowing four runs. Sveum said there has been no talk about moving the right-hander to the bullpen.