Hmmm... I'm currently working on a scarf with Boucle yarn too on metal needles and I don't have this problem
They seem to slip very well on metal needles... I guess you might be knitting too tightly?
Here, you can take a look at mine:
...and a close up if you need to see more detail about the yarn, needle, or tightness of stitches.
That's all I can provide...
maybe not of much help