Yeah, that's a bit finicky, but what works for me every time is: After talking with Tarish, visit Ulcro. Keep in mind that if you fail a roll, you will need to give him a present (like a sword from your inventory). After you are done talking with him, start exploring the camp repeatedly. One of the events that you are going to get is about looking for Delane, then you have to get a successful roll and you will find her tent. After that wait for the start of the next week and you will get the option to sneak into Delane's tent and to talk to her (subject to a successful roll). While talking always choose to gain her trust and after you run out of dialog options, you will get the possibility to plan her rescue from the main camp menu (maybe from the next week). It should be clear from there, though beware that both the planning and the rescue itself require a couple of successful rolls.