Before talking about complicated things that about a tenth of the players actually know:
Check all the following points:
_ The refinery is a supplier of the daily goods factory: that means that in one factory's window there is a link to the other.
_ The refinery has oil in stock so it can produce plastic
_ The daily goods factory is not already full of plastic (impossible unless you already transported plastic to it)
_ At each factory, the station is connected to the factory: the factory is in the cover area (show with key "v") of the station, you can check it in the station's details
_ At each factory, the station can accept goods: a good mini icon (in pak128 it looks like a lump of shi...something) is displayed in the station's windows. If it cannot, you just have to add one building which is able to accept goods to your station (a building which has the same icon on its toolbar button).
_ Your train can transport plastic, which is of the category "piece goods" (or something like that).
If you don't find the problem, please upload a screenshot with your convoy, the window of each factory and each station opened.