okay you've got two options.
Modify your extruder or switch to polyflex.

The only colour of ninjaflex that will reliably print on an unmodified mk8 extruder is green. Nothing else I've tried will work.

Polyflex is the best flexible filament for printability.

The best rubber filament I've used is flexismart.

For any flexible filament you need to reduce the print speed to a MAXIMUM of 30mm/s (ignore what it says on the packet if it says you can print up to 50mm/s) And run it at the top end of the recommended print temperature. Loading can be an issue as it loads much faster than it will print at.

The trick is to stop the load as soon as you see the filament come through the nozzle.
Sometimes I find I have to unload and reload a couple of times to get it setup properly.

Polyflex, just works. But is a flexible plastic (pla) NOT a rubber based filament

The most reliable tpu (rubber based) flexible filament I've used is flexismart. Runs at 210 and 30mm/s without any problems.
I'm just about to order some more, which will be the first time I've found a rubber filament reliable enough to actually use a whole roll up !
It's not quite a s strong as ninjaflex - but it's a helluva lot more user friendly.

Sebastien has used a fariyl simple modification on his printer and reckons it prints ninjaflex really well now.
Used one of the little plastic ptfe tubes that sits inside the heater block. You insert it between the top feed hole oin the extruder and the feed hole for the heater. carefully cut out the section by the feeder/hobbed bolt so the teeh can still engage the filament and the tube doesn't get caught by the teeth.
Essentially it stops the filament from flexing and getting wrapped round the hobbed bolt and redirects it straight to the heater block.

Simple and ingenious :-)