How can I resolve the issue of tapered sections not being displayed in the model? 

In the creation of any tapered section, one thing has to be noticed that the number of vertices should be the same at i-end & j-end of the tapered element. To understand this in a better way please refer to the attached picture and following link:- https://midassupport.jitbit.com/helpdesk/KB/View/5263275

Another point, just check off the "Merge Straight Lines" option in SPC while creating a section to avoid any merge of nodes.

After importing the sections with the same no. of vertices at both ends, there would be no such warning.