Ok so...
This is not only a recall issue....
Treats toys pate etc are all using operant conditioning eg if I do something this will happen. So consequence to a behaviour. At the moment the consequence does not rate up there with playing with another dog
You need to classically train the recall - a whistle on its own is not a magic tool to make recall wark for a dog but if your classically condition it - it will be.
Think of other things your dog may have learnt without you teaching them eg my dogs will all run into the kitchen if I open the fridge (never taught them that they just know food may fall on the floor, they always run up to me when I pick up their leads). This is classical conditioning something happens and it always happens.
So pip on the whistle with the dog close to you in a quiet environment in your house immediately give the dog a treat do this over and over again. Always have the dog next to you. pip and food, pip and food say and ask for nothing. If you blow the whistle they must get food.
Slowly and gradually you will build up the distance eg have the dog the other side of the room, in another room, have treats on the side and the dog still comes to you. have the dog in the garden. When the dog consistently comes to you with the whistle in a quiet area you can add in distractions DO NOT blow the whistle when out and about when you know your dog will not come to you. Total Recall does this method and will break down the stages for you.
It works with all dogs if done correctly and at the correct pace for the dog.
However I know you had a shy rescue but you do not want your dog interacting with every dog they meet. Some dogs they can sit look and you and then their reward is to be released to meet the other dog if safe to do so and agreed with the other owner. Other dogs you recall your dog to you and have a game with each other as the other dog passes. Some dogs you ask your dog to sit whilst the other dog wakes passed.
You do not want your dog to see another dog and always interact with them.
Just steps onto soap box to say no dog or breed is stubborn, they may be distracted by the environment, owner asking them to do behaviours that are not train or inconsistent training. I am aware that many owners disagree with this but I will say it anyway
with science and education on my side!