I have bought materials from Ultimaker, which are PLA Flexible white and PLA Flexible black, however, I'm having trouble using these materials. Every time i use them and as i start printing, the material get jammed and stuck in the material feeder from the back. I have asked about it and have been told to use vegetable oil. Then another source told me not too since it might damage the machine. I hope someone could answer me on how to use these materials without having trouble!