In Miami, the American team introduced its first major upgrade of the season in the form of a revised floor.
Helped by issues for others and a red flag in Q3, Kevin Magnussen qualified a surprise fourth.
The Dane made a bad start and, after doing his best to make it hard for rivals to pass – notably Ferrari's Charles Leclerc – he eventually finished 10th.
Meanwhile, his team-mate Nico Hulkenberg fell from 12th on the grid to 15th at the flag.
Steiner said that the new floor, with the changes focussed on the underside rather than the more visible edge, had been an improvement.
However, he reiterated that the team has to improve the car's performance in traffic.
"It seems to be working," he said of the floor. "We just need to find out what we need to do to get more stable in traffic, that you're not wearing the tyres out this much in traffic.
"Because the guys say, in traffic, they can see the other cars aren't struggling as much as our car, so the guys in the aero department will work on it."
He added: "We have got some parts for Imola, some front wing flaps, not complete wings. And they keep on coming."
Source: Autosport