The problem might be Panasonic installed 35kWh of capacity years ago.
I don't think this is accurate: just today Panasonic said that they have completed expanding GF1 to 35 GWh/year
on 2019 March 31, and last year Tesla reported cell output capacity at around 18 GWh/year, before the Panasonic expansion (which involved installing new lines).
Here is a Tesla's Q2'2018 10-Q:
"At the end of July [2018], Gigafactory 1 battery production reached an annualized run rate of roughly 20 GWh, making it the highest-volume battery plant in the world by a significant margin. Consequently, Tesla currently produces more batteries in terms of kWh than all other carmakers combined."
So Tesla and Panasonic almost doubled GF1 capacity within about 6-7 months, from 18 GWh to 35 GWh.
This is how Tesla characterized their GF1 expansion plans back in the 2015 10-K:
"By the time the Gigafactory reaches full, annualized production in 2020, we expect battery pack production capacity to reach 50 GWh. Of this, we expect to build 35 GWh of cell production capacity at the Gigafactory and purchase 15 GWh of cells from other manufacturers, potentially including Panasonic."
So the 35 GWh GF1 target and intention to use non-Panasonic cell suppliers dates back to 2015, and their deadline for that was 2020, not 2019...
They dropped this language from 2016 and later 10-K's - i.e. they went more opaque about their Gigafactory expansion plans.