Flex filament underprints and fails constantly

[TopHat]

Hi,

I've searched and tested and can't really pin point the problem.

Printer: Zortrax M300 Dual. 0.4mm nozzle.

Filaments: Zortrax Z-Flex, Fiberlogy Flex 40D, F3dFilaments TPU

Issues: Zortrax Z-Flex and Semi-flex print allright, there are small underprints but the outcome never fails and is acceptable. However any other flex filament starts off fine, then little underprints appear and in the end when prints are bigger than several hours worth of printing time filament jams, printer starts to "print" in the air. Every other type of material prints well.

I've run through various settings, broad range of temperatures, print speeds, everything. I've changed the nozzle, disassembled and reassembled the feed mechanism. I think Z-flex is a bit more rigid than other TPU based filaments but other than that I can't say what's different about them.

Here are some photos of a recent failed print:

20230621_101122.jpg20230621_101117.jpg20230622_072651.jpg20230621_101133.jpg

Tips and guides appreciated.