(My question has been asked here before but with no working answers that I can see. E.g. Xcode4 Workspace with Static library project & application project)
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I had the same issue with my team. One of the developers was suffering with this issue, however my xcode was able to compile and found the header properly. BTW: all "build Settings" were properly configured (always search user paths, user header paths, etc).
I realized that his project was in a directory with spaces in its path (../my project/blah.xcodeproj). Changing that, Xcode was able to find the headers from the static library within the same workspace.
Just be aware of directory names. My two cents