Tesla's third-quarter delivery numbers were both impressive and depressing. The carmaker is on pace to sell 100,000 vehicles annually for the first time in its 14-year history. But it's also far, far behind with the production of its new Model 3 sedan, the vehicle that's supposed to bring Tesla to the masses and spell the beginning of the end for gas-powered cars.

Tesla said it would produce 1,500 Model 3s in September — it has produced fewer than 300 since launching the car in July.

Getting to 20,000 in monthly production by December now seems like a hopeless expectation, as does CEO Elon Musk's prediction that Tesla will manufacture 500,000 vehicles annually by the end of 2018.

This means the half-million preorders for the Model 3 could go unfulfilled for several years, putting many $1,000 refundable deposits for each new car in doubt. That threat is real, but the markets are unconcerned. Tesla stock is still up 65% in 2017, and the brand has lost none of its captivating aura.

But a carmaker that has been around for as long as Tesla should be good at something that Tesla clearly isn't: building vehicles.

So why is Tesla struggling to build the Model 3 on its own admittedly ambitious schedule? There are five main reasons.