Not sure what island you're on, but there was a recent event mainly on Beluaterra that changed how portal stones work.
In-character, the aether between worlds has been destabilized by so many portal stones being activated over the past few years, and now doing so is dangerous.
Out-of-character, as Chénier says, portal stones were never intended to be such a common thing (except during a couple of invasions).
The inventory page gives a pretty clear warning about the difficulties and dangers of using portal stones now.
