I have seen this too. Seems to happen when you place two or more blueprints, when at the placement there are not enough raw materials to build them.
If it is only one blueprint it seems that construction get started when the needed raw materials are produced.
But when two or more blueprints are waiting, then sometimes they are stuck never to start construction, no matter the amount of raw materials.
It is easy to recover from: Just delete the stuck blueprints and place them again, and they are good to go!