I've have been having trouble printing flexible filaments on my Wanhao Duplicator i3 Plus and Monoprice Maker Select Plus 3D printers, so I decided to try out a mod/upgrade to see if I could resolve the problem. It was a great success, in the end.
The trouble is that flexible filament would bend and go through any cracks that it can find. So if the is the slightest gap anywhere from the extruder wheel to the nozzle, it will curl up and go out that way, rather than enter the hotend. But a quick 3D print and a bit of PTFE tube can easily solve that problem.
STL available here: https://www.thingiverse.com/thing:1340811
Music: Rounds by William Rosati