The Ducati rider lined up fourth for Saturday’s race before dropping five positions on the opening lap.
He then ran wide at Turn 15 to drop more positions, eventually finishing the race behind team-mate Enea Bastianini as the lowest-running Ducati.
Reigning world champion Bagnaia said he felt like “something was not working well” during the earlier warm-up session, with the bike “moving a lot”.
He said the team doesn’t yet know what the issue was, but that he was “quite sure” that it didn’t come from his set-up.
Speaking after the race, he said: “Sincerely, my team is already working and trying to understand, checking on the data what happened.
“This morning in the wet I was second in FP2, in qualifying I was fighting for pole position.
“Then already from the warm up lap I could understand something was not working well, and it was quite difficult to do everything. Riders were overtaking me from the inside, from the outside, and I was a bit on the limit going slow like this.
“It was very difficult but we know our level, we know that 15th is not our position, it’s not our target and we will work for tomorrow.”
He continued: “[The bike] was moving a lot. In the warm-up lap I just tried to warm the wet tyres and I went wide almost everywhere.
“I was not understanding what was happening, I was just trying to be focused and trying to push from the start but already when we started the race and I did the first braking I almost crashed the first braking.
“Then I was risking a lot all of the race, 10 laps of risk.
“I don’t know what happened but it was the bike of the crash, so maybe we have to check for that, but I’m quite sure it was not the set-up.”
Saturday’s sprint race was the first Bagnaia has not scored points in, and the first race he has not scored in since the French GP in May.
He had a similarly difficult outing at Silverstone last year, having finished Friday’s running in 11th, slightly improving on Saturday morning to finish FP3 in seventh place before qualifying in fifth for the race.
Bagnaia went on to win the race ahead of Aprilia’s Maverick Vinales, who took his first sprint podium on Saturday, and then team-mate Jack Miller, now riding for KTM.
The Italian is optimistic he can turn around his fortunes again this year, and said: “Yesterday afternoon I was feeling great with the used tyres, new and used tyres I was quite competitive.
“I think in this track it’s more difficult than other tracks to make the difference, because you have to consider more the rear tyre consumption.
“It’s very important not to push too much or in the wrong moment so I think we have the package, we have the performance to be in the front, to fight, and in both cases dry or wet I think we can fight for the top positions.”
He added: “I think yesterday we saw that many riders have the potential to be competitive, because like I said before, it’s not the performance in one lap in this track that makes the difference, it’s the performance in many laps and if you have the pace constantly, not the fastest but more constant, you can have an advantage across 20 laps.
“I think our potential yesterday was quite high, we were controlling well the tyre consumption and this is for sure something that helps us, but I think that many riders are in our situation, so let’s see.
“Last year, if you remember, I think 10 or 11 riders finished in three seconds, and this is something which normally doesn’t happen. So this is a particular track.”
