Yamamoto concluded the last weekend's opening event of the 2022 season at Fuji Speedway with a pair of 14th-place finishes, leaving him with zero points.
That followed a truly miserable return season with Nakajima in which he ended up 13th in the standings with a best finish of sixth, incidentally at Fuji.
Following Tomoki Nojiri's dominant title campaign last year, Yamamoto vowed to use the success of his Honda stablemate as fuel for his own comeback.
But with his troubles stretching into a second season, and teammate Toshiki Oyu also suffering a difficult weekend at Fuji, Yamamoto admitted that the atmosphere within Satoru Nakajima's eponymous team is not good.
"It looks like the same situation as last year, but honestly for myself it’s a difficult feeling," Yamamoto said. "Last year it was only me who was slow, Toshiki was so quick, but this year both drivers are not good and the speed is about the same.
"It’s a different situation [to last year], but we are struggling this year too. It's a difficult situation.
- Motorsport.tv is showing every race of the Super Formula season (available worldwide except Japan). Click here for further information.
"Racing is always up and down and it’s difficult to always keep the same high level. Currently, Tomoki [Nojiri] is really quick and is keeping a really high level. He is a great driver, so my target is him, but he is too far away at the moment.
"The motivation is difficult to keep up, but it’s not only the case for me, also for the engineers, mechanics, Satoru [Nakajima]-san. For the next race I have to change my mind, but it's not easy."
Yamamoto was ninth of 10 drivers in his Q1 group in Saturday's first race at Fuji, leaving him 17th on the grid for the season opener, a position he would repeat in second qualifying on Sunday.
While Oyu fared even worse in qualifying, lining up 21st and then 18th on the grid, Yamamoto ended up behind his teammate at the end of both races, as Oyu charged through to seventh on Saturday before finishing 11th the next day.
"We had no speed and the balance was also bad," continued Yamamoto. "The straight-line speed was also bad, although we don't know the cause.
"[For the next race at Suzuka] we are aiming to be in a higher position, but we're stuck right now, so I want to work hard to get out of this situation."
