Going off the top of my head here, but I think weak linking was introduced in 10.2, so you should probably set the deployment target to 10.2 or later. Also, if you are using weak linking, make sure you point at the SDK you want to take advantage of.
Also, are you weak linking frameworks, libraries, or explicitly weak linking functions in the code? Of course, if it's your code, this should really be in the Developer forum.