So as far as there is no easy solution, let*s try with some sophisticated.
There are two major reasons for too less progress of the filament: mechanical blocking or wrong software settings.
For the mechanic you may check if the filament is moving smoothly and without hinders through the tube. Relax the extruder counter guide by hand and move the filament by hand through the PTFE tube. This should be possible without too much force.
I the stepper motor itself is running free, then there should be no issue with mechanics. However, if you hear a clicking noise coming from the extruder while pushing the filament, you have an issue with mechanics.
To be on the save side, you may check the current settings of the stepper motor driver as it is describeg in the WIKI.
For the software settings there may be some wrong value for the step counts/mm for the extruder.
In the “CONTROL” menu, submenu “Motion” there is an entry for the value “ESTEPS/MM”. For the original Vellemann FW you will find a value of 200; for working with a 100% flow setting in your slicer you may change this value to approx. 150. Any other value indicates a wrong firmware condition. Remember to recall the “restore failsafe” settings and store them in the EEPROM.
If even this doesn’t help please check your firmware.
I hope this will help you to as a first attempt, otherwise you may gather some other findings which may lead to a solution.