The manufacturing is the time consuming part of new bits rather than the design. This is can be the difference with the big and small teams. (Gary Anderson mentioned in a question here that top teams put a lot of effort in reducing manufacturing times
http://www.autosport.com/premium/fea...-than-mercedes )
Big carbon fibre parts can take days to build-up and possibly can only be 'cooked' one at a time.
It's not just Renault this year that has had these problems:
http://www.autosport.com/news/report.php/id/129434