Over the past couple of weeks the embered are having issues with either summoning players into their worlds, or accepting invitations to phantom into another players realm.
The reason for this is a change to the way the leveling system that dictates which worlds you can and can not enter. The last update that was added to Dark Souls III brought several listed changes, however developers From Software are not very forthcoming when it comes to providing patch notes and users are left generally relying on the hard work of data-miners on websites such as Reddit.
Here is how you can fix that problem:
If you are unable to view the video, here are the details for you:
If you spend enough time in the game though and experiment with different tactics you will find that there has actually been a change that makes your weapon level relevant when trying to summon/be summoned where as previously if was only your characters level that was important.
If you are experiencing this issue in Dark Souls III you will now need to start leveling up your weapons. Once you do this you will be able to get that soap stone back out and begin invading, or assisting others again.