Areva has a lot of trouble hiring new engineers. Nuclear power is not as popular in France as it once was. Also, you don't only need engineers: you need lots of industrial workers (such as welders), which, because of deindustrialization, are not that common in France anymore.

It's not only the loss of knowledge, it's also the higher security standards. The last reactor being built, Flamanville 3, was supposed to be ready by 2012, is still being built and not estimated ready before the end of 2022.