A while back blizzard made a change to Soulstones that took them off you as soon as the warlock who applied it left your party or raid. If you really need a soulstone for something without having a warlock in your party you can join any battleground and get one from a warlock that is not on your server. When you leave the battleground the soulstone will remain on you for the full duration.
Has it's uses and most warlocks are nice enough to throw it on you if you just ask.