I think they are both having trouble scaling up different parts of the production line.

So far, it seems that Tesla has been focussing quite a lot on getting great at producing batteries, including the logistics for that. Which is what they seem to be good at (given that they produce industrial size battery stations and batteries for home use as well). But they do not have as much experience in managing a mass production line for cars.

BMW on the other hand has a lot of experience in mass producing cars, but they do not have as much experience for batteries. The hardest bottleneck for BMW might just be getting the batteries themselves. Tesla has worked hard on that by building their Gigafactory for example.