Oh, the dreaded fuchsia problem. It's caused by buying a bad batch of dye. Not all dye vendors are equally likely to sell bad batches of fuchsia, and some are better than others about taking returns on defective dye. Only fuchsia and mixtures made containing it have had this problem, that I've heard of.
Heating MX dye will cause it to react with the water, so that there is no reactivity left to bind to the fabric. I did that once. My purple LWI mixture came out blue.
Try adding just a tiny drop of Synthrapol or hand dishwashing liquid detergent. Adding urea (up to 9 tablespoons or 100 grams per quart) may increase the solubility a bit. Stir it a lot, leave it for an hour, and come back and stir it some more, or, preferably, use a dye-use-only blender to mix it (impractical if you're not planning to do a lot of dyeing). After all that, strain out the undissolved particles through a coffee filter or the toe of a pair of pantyhose, held in a strainer (which is not to be used for food afterwards).
Also see the entry in my dyeing Q&A weblog for
July 13, 2005.
Paula