Concerning the OP, I suppose it depends on what you mean by "major problem". In my opinion, I think Jihadism and lone "mass shooters" are a far bigger problem. And I think our governments resources have been rightly prioritized to address these threats.
That being said, I do think White Nationalism is both dangerous and on the rise. I feel the same way about WN as I do about mainstream Islam - that is to say; the vast bulk White Nationalists are not terrorist. But it's unimaginably problematic that they often provide cover and fuel to those who do carry out these acts of hate, harm, and murder.