In Italy, could only salvage 10th after taking a power unit grid penalty hit.
In the previous race in Belgium, Norris lost fifth place with a last-lap engine issue, while Sainz retired early.
In just two weekends, rival Renault has made a net gain of 25 points on McLaren in the battle for fourth place in the championship, the Enstone team having earned fourth and fifth places in Italy and outpaced McLaren in qualifying at both races.
"It's important now as a team to go away from here, re-group, reset, analyse what happened in detail on the pitstop side, and then hopefully putting more downforce on the track we're back to where we've been before the shutdown in terms of being competitive," said Seidl.
Seidl said that until the Monza issue the team's pitstop performance has been good thus far this year.
"We still need to analyse in detail what happened. It was quite a messy tyre change at the front right corner, which then led to the wheel not being fully tight. We realised straight after we had to abort the car. Unfortunately these things happen.
"Still, compared to last year I think we've made a great step forward in terms of pit stops and so on as a team, we had great races also where we made points, and overtaking people by doing good pit stops.
"It's important now to go back home and analyse in detail what happened, and make a re-set and get back in Singapore to the for we had before the shutdown."
Seidl was encouraged by the performance of the latest Renault engine.
"We all now have to C-spec in the car, which is working fine, working well. It was a small step in terms of performance and also reliability.
"I don't think there is anything which is holding us back, just good to focus on ourselves and make sure we keep going as a team and keep developing.
"These two races have been a setback now, after having a good run before the summer break. It's part of the sport, part of the development also I would say as a team. It's important now as I said before to do a re-set again and keep going again from Singapore onwards."