Some bone-jarring tackles took their toll on the Wanganui backline, already struggling for cohesion in the wet, while Horowhenua-Kapiti maintained solid control of the ball for phase after phase of near risk-free carries around the ruck.
Eventually, Wanganui would over-play their hand in front of Waikato referee Michael Winter and So'oailo did the rest - slotting five from eight attempts, with his misses being long-range efforts from near halfway.
Leading 8-0 after 30 minutes and having tried to play some attacking football, mostly denied by strong Horowhenua-Kapiti defence, Wanganui's only real chances in the second half were a strong 20m drive from a penalty lineout, which was eventually turned over, and a couple of desperate breakouts which were snuffed out just past the halfway.
All the playmakers in halfback Lindsay Horrocks, young first-five Cody Hemi, goalkicking fullback Nick Harding and centre Craig Clare, until he was helped to the sideline, made a composed start.
Yet by the second half they were pushing silly passes, losing possession coming into contact, kicking out on the full and resorting to chip-and-chase in their own half.
The tryscorer was winger Jona Sawailau, who crossed in the far corner after a neat interchange between No8 Bryn Hudson and second-five Timoci Seruwalu put winger Cameron Crowley through a gap, leading to a penalty scrum opportunity from 5m out.
However, Sawailau did not adjust his one-handed running style for the conditions, with the breaks by himself, Crowley, Seruwalu and others often coming unstuck.
Hudson had a very strong first 40 minutes with his carries, but infringements proved costly in the second half, lock Sam Madams a culprit, as the Wanganui lineout also struggled in the wet with reserve hooker Cole Baldwin missing his jumpers.
It was as co-coach Jason Hamlin had feared, with his team's struggles playing right into Horowhenua-Kapiti's hands with their solid set piece.
Wanganui was also desperately unlucky that several of their own attacking kicks would bounce their way but were spilled when a try was on - literally every 50/50 moment went to the hosts.
"On the way down we saw that [rain] and how we can adapt to it," said Hamlin.
"We didn't use it the way we liked [in first half], got one try.
"All credit to them, our disciple let us down. We were lucky we got one bonus point.
"It wasn't our players best day, but they were trying under pressure.
"The catch and pass skills weren't quite there."
Captain Roman Tutauha felt his side lost the game in the "one percent moments".
"Biggest thing, I think, was discipline at breakdown time.
"It just gives them piggy backs, keeps them believing."
"It's going to rain, rugby is a winter sport, got to play these wet weather games."
Once again, the narrow loss dropped Wanganui down to fifth place on the table, but the Meads Cup playoffs can still be secured if they defeat fourth-placed North Otago at home this Saturday, while hoping results from other games go their way.
"We said [Horowhenua-Kapiti] was the biggest game of the season, but next week has to be," said Tutauha.
Hamlin said the team will try to address its faults, but the reality is on a dry-track this is still the competition's form team - they just have an achilles heel in the pressure of boggy, wet-weather rugby.
"What we are doing won't change - it's how we adapt."
Horowhenua-Kapiti 15 (James So'oailo 5 pen) bt Wanganui 8 (Jona Sawailau try; Nick Harding pen). HT: 8-3 Wanganui.