Why can't I use the engineers??

I don't get it.

I had a fine game and used a lot of great engineers for wonder-building - The last two of them couldn't add to the production and I had to use them as factory-builders - which sucks since it's so late in the game...

What did I miss? Is there a limit of engineer-rushing or something?

You cannnot use them on projects ever.

It looks like your building the Manhattan Project. Projects can't be built with GEs.