Off the back of two race wins at Albert Park earlier this month for Mostert, the Walkinshaw Andretti United squad failed to fire in today's three-part qualifying session at Wanneroo Raceway.
Mostert was just 23rd fastest while Percat was dead last and will start tonight's opening sprint race from the rear of the grid.
Following the session Percat made it clear that he was last on pace, adding that he wasn't sure why his car is so slow.
"It's just bad," he said. "That's legit for both of us, that's it. I'm not really sure what to say.
"It doesn't feel like I should be last. The lap time just comes on the dash and you go, 'alright, that's us'. We've got a lot of work to do.
"We've got a great group of engineers here. We're all pushing to get it done. It's just hard. We'll dig deep, this team always does."
Mostert, meanwhile, blamed his lack of pace on what he called a trend in the car that has worsened significantly on the unique Wanneroo layout.
"It's a little bit of a weakness here this weekend," he said. "Kind of what we've had for a little bit of a trend, but for whatever reason the track this weekend is really highlighting quite a big issue for us.
"The whole crew here at WAU is trying really hard to try and turn it around. Two-day meetings make it pretty hard. If we had a three-day meeting, Friday we'd know the problem that we have at the moment and we would swung a bit harder today.
"I know I've got the team behind me, we're trying to make this car fast. Not an ideal day for us, but we'll put our head back into the trailer, see what we can come up with for the race tonight.
"What do you do? Being upset about it is not going to bring us down. I love this team, they've given me some fantastic race cars in the past. Some days you get it right, some days you get it wrong.
"We need to keep the positivity up and go out there and fight hard tonight."
The 46-lap sprint race kicks off at 6:40pm local time.
