A pigtail is a short, curved, and very appropriately named piece of pipe that sits under a gauge or a pressuretrol. Its first job is to fill with water and keep the latent heat of steam from reaching through to the gauge or the pressuretrol. This increases the lifespan of whatever the pigtail is serving.
The pigtail’s other job is to clog. There’s only one way in and out of this thing. Roiling water is under it and a lot of surface debris is flying up into it. Sooner or later, every pigtail clogs. Your time is too valuable to spend cleaning it, so just replace it. And watch for the common situation where two pressuretrols (one backs up the other) share the same pigtail. Sharing a pigtail was some knucklehead’s way of saving a buck, but when that pigtail clogs, both pressuretrols go offline with it.