Try soaking the tip in some warm water for about 10 minutes, then scribbling with it. If that doesn't work, you can try nail polish remover. Just the tip. It doesn't always work, but usually does.
Sometimes holding the flame of a lit match to the tip for a second works. But only a second or it can melt the plastic pen barrel! (PS I have a handful of white pens that won't write. Have tried so many brands and still don't have one that I really think it worth the cost. Which brand do the rest of you prefer?)
I've never found one I really like, either. I do have a paint pen to go over some things that I need white. I stamp it, then go over with the paint pen. It's too thick for writing, however.