Steel knitting needles sometimes need to be reground. The best tool that I have found for that is the little ceramic sharpening stone that Felco sells to sharpen its pruning shears. It has a groove that runs down one side of its generaly triangular cross-section. The bottom of the groove is rounded and makes getting a nice rounded tip easy. I use a bit of WD-40 as cutting oil
Do not try to regrind aluminium needles.
Aaron