Look in the spec if the diameter DN and OD of the pipe is the same that in the elbow. If the DN or OD is diferent the Plant3D cannot to connect this items.
In the case of plastic you need to create new fittings and new pipes because DN is don't use in plastics, is used OD. There are for example DN100 and for this diameter there are two diferents diameters outside OD110 and OD125. That's why in the catalog you need to change DN and OD to the same value in all components.
Regards
