He simply hasn't been able to put away Oakland.
His problems continued on Wednesday night when allowed three runs, including a tiebreaking two-run homer by Sam Fuld, in the ninth inning and Athletics rallied for a 5-4 win.
Qualls has 3.55 ERA, but has allowed 11 runs combined in six appearances against the A's.
"I don't think I've ever really had an opponent that's been as rough as the A's," he said. "I feel like I've been throwing the ball well this year, but every time the A's come to town they have my number. You look back on it and I thought I threw the ball pretty well and the scoreboard just doesn't dictate that at all."
The A's trailed by one entering the ninth before Eric Sogard's RBI single, and Fuld's shot off Qualls (1-4) landed in the seats in right field to put Oakland up 5-3. Fuld, who entered on a 0-for-9 skid, finished three hits, including a double.
"They're a really good team and they've got good players," manager Bo Porter said. "(Wednesday) they came back and got us."
Houston trailed 2-1 in the seventh after Coco Crisp's solo home run in the top of the inning, then tied it on Robbie Grossman's RBI single.
Jose Altuve's big league-leading 181st hit put Houston on top 3-2. The groundball single rolled just out of reach of a diving Alberto Callaspo and into shallow right field. Altuve also swiped two bases, giving him an AL-high 49 steals.
Jonny Gomes' sacrifice fly put Oakland ahead 1-0 in the sixth.
Oakland starter Drew Pomeranz, recalled from Triple-A Sacramento before the game, allowed one run — none earned — and three hits in 5 1-3 innings in his first major league appearance since June 16. He struck out seven.
"We just had trouble hitting his curveball," Carter said. "He was throwing it for strikes and we were just not getting hits on it."
Houston's Brad Peacock also lasted 5 1-3 innings, allowing one run, four hits and five walks.
Carter has 33 homers this season. He has homered in three straight games and has hit 20 home runs since July 1.
Athletics: Manager Bob Melvin said he expects SS Jed Lowrie (broken right index finger) to be ready for an injury rehabilitation assignment this weekend if there are no issues the next two days. INF Nick Punto (hamstring) is getting better but is behind Lowrie because he still isn't able to run at full speed.
Astros: Reliever Darin Downs (strained muscle on his right side) is to start a rehabilitation assignment with Double-A Corpus Christi on Friday and is expected to pitch there on Sunday.
The Astros open a four-game series against Texas on Thursday, with Collin McHugh scheduled to pitch against Nick Tepesch. Oakland travels to start a four-game series against the Los Angeles Angels. Sonny Gray opposes C.J. Wilson in the opener on Thursday.