While these are all good points no one has mentioned simply scale up. Everything on the design sale or in R&Dis on a smaller scale and transfer to manufacturing is just difficult. If parts are being machined machines have to be found that can create small parts in duplicates, they need to be well maintained. If the product requires a chemical making sure the exact amount is being added each time just the general scaling up process.
Also neb2 mentioned some good points to successfully transfer a design and one more to be added would be to spend more time early on thinking about how this project can me manufactured efficiently and effectively.