Flexible materials, like Ninjaflex or TPU, should only be printed using extruders that are specifically designed to handle them. The main reason for this is that the extruder must have a fully constrained filament path to work well with flexibles, otherwise, the filament will just fold over and bind up inside the extruder (imagine trying to push a wet noodle instead of a rigid stick). The Ninjaflex FAQ has more information about specific extruder requirements.
Here is a list of printers we sell that were designed with flexible materials in mind
- Lulzbot TAZ 6 + Flexystruder (this is our top recommendation for printing flexibles)
- Ultimaker 2+
- Ultimaker 3
- Rostock MAX v3
- BCN3D Sigma
If your printer is not on the list, then it will probably have a lot of trouble printing flexible materials. If you wish to try anyways, here is a trick that may help. You may also be able to modify your machine by installing a Bondtech QR extruder.
See this article for more general information about filament compatibility.