Cahill allowed six runs on eight hits and lasted just five innings to raise his ERA in June to 9.30 and he is now 0-4 during the month with one start left on Sunday against the Braves.
"He can't locate with any consistency and it gets him in trouble," D-backs manager Kirk Gibson said. "He's been inconsistent recently, so we'll have to look at the video and see if we can get it fixed."
With Cahill's struggles, the D-backs' starters winless streak extended to 17 games. The last time a starter picked up a win was June 5 when Wade Miley beat the Cardinals.
"We've got to play better early in the game," Gibson said. "We've got to pitch better. We dug ourselves a hole. We couldn't dig out of it."
The D-backs missed a pair of early scoring opportunities against Nats' starter Gio Gonzalez.
Willie Bloomquist led off the game with a double, but was stranded. The D-backs had a pair of singles in the second, but Martin Prado was thrown out at the plate by center fielder Denard Span and Arizona came away empty.
"We had some opportunities early," D-backs first baseman Paul Goldschmidt said. "It looked like [Gonzalez] didn't have his best stuff and then he locked it in for three or four innings and then we were able to get a few more baserunners there right before they took him out. But he did good job of throwing strikes, threw his fastball to both sides of the plate and his curveball you know is one of the best in the game. [When] a guy like that gets ahead, it's going to be tough."
Cahill cruised through the first two innings, but the third was a different story.
After a leadoff walk to No. 8 hitter Kurt Suzuki and a sacrifice bunt by Gonzalez, the Nationals collected four straight hits off Cahill. The final one was a three-run homer by Adam LaRoche that gave Washington a 5-0 lead.
"To be fair, he had a couple of ground balls that found ways through as well," Gibson said. "But the leadoff walk to Suzuki, it just put a bad spin on the whole inning. It just wasn't a very good inning for him."
Given his command issues, Cahill was asked if he felt his mechanics might be the issue.
"I don't know," he said. "I always feel like my delivery changes a lot, from start to start and from inning to inning. I wouldn't say it's changed too much more. But for some reason even when I get ahead I make a bad pitch. That's what really hurts."
D-backs catcher Miguel Montero has noticed that Cahill's arm angle seems to change.
"I don't know if he's trying to change his delivery or what, but it changes from pitch to pitch," Montero said. "Sometimes it's over the top, sometimes a little bit lower, so it's hard to figure it out. Because he's a guy that doesn't need to do too much. He just needs to grab the ball and throw a good sinker down the middle and let it do the job."
Cahill said that he did not experience any issues with his right hip. In his last start, Cahill was forced to leave after one inning when he was hit in the hip by a line drive.
When he throws it near the strike zone, Cahill's sinker is one of the best in the game, but he seems to have gotten away from that pitch a bit and his other weapons have not gotten the job done.
"For some reason, I've noticed I make a lot of mistakes on my changeup," he said. "Just kind of leaving it up, middle. I don't know if it's me trying to do too much with it or whatnot, but I think the biggest thing is just concentrating on the good one I throw and not trying to overthrow it or whatever because that's usually when it's up."
Down 6-0, the D-backs finally broke through against Gonzalez in the sixth when Aaron Hill doubled and scored on Montero's single to right.
Arizona cut the Nats lead to 6-3 in the seventh on a Paul Goldschmidt RBI single and when it capitalized on an error by Anthony Rendon to score a pair of runs.
The D-backs played the game with a 23-man roster as pitcher Ian Kennedy and infielder Eric Hinske sat out due to suspensions imposed by Major League Baseball for their part in a June 11 fracas.
Hinske will return to the roster Wednesday, while Kennedy's suspension ends Saturday.