The FBI are plainly successful at targetting Moslem terrorists, but the planning of terrorist acts by Moslems remains frequent.
It's the same situation in the UK. Our security services report that there are large numbers of terrorist plots among Moslems. Once in a while, one escapes notice and the attack happens. The good news is that our security people have thoroughly infiltrated the fundamentallist communities and have widespread contact networks among law-abiding Moslems.
This isn't to say there aren't white nationalists as well. But they are likely to be lone wolfs and so harder to discover in advance.