I'm not on LU but if they are air transport corps you have to have an airport built first which while done automatically when the corp is ordered due to supply chain issues is the usual hold up.
unless it's the production plants themselves.
when a corp is ordered built, it will order a production plant. IF there are production plants in stock then it will build next game month. If none are in stock then it has to wait until the production plant ordered comes in so there is one in stock. If you are building a lot of corps in a country it's easy for things to fall behind if production plants are in shortage.