AGSQueryTaskDelegate not firing

Question

I am trying to query all the features from my database through a querytask, but no delegate methods are being invoked.
I created a strong property AGSQueryTask :@interface MyClass : NSObject <AGSQueryTaskDelegate>

@property(nonatomic, strong)AGSQueryTask* queryTask;

set the delegate as self: self.queryTask.delegate = self;

but nothing happens.
Ideas?

NicoleDe_La_Feld · Accepted Answer

Thank you for your answer. I already did what you say and it's right. 
The problem was that I accidentally deallocated the queryTask before the operation was completed.