cachedResponseForRequest not called

I have subclassed NSURLCache trying to see how caching works. In my app delegate, have this

NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:0
                                                            diskCapacity:50 * 1024 * 1024
                                                                diskPath:nil];

[NSURLCache setSharedURLCache:sharedCache];

My requests are setup like so

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:someUrl
                                                           cachePolicy:NSURLRequestReturnCacheDataElseLoad
                                                       timeoutInterval:10];

But in my NSURLCache subclass

- (NSCachedURLResponse *)cachedResponseForRequest:(NSURLRequest *)request

never gets called. However

- (void)storeCachedResponse:(NSCachedURLResponse *)cachedResponse
                 forRequest:(NSURLRequest *)request

Does get called.

The headers for my request responses look like

"Cache-Control" = "private, max-age=86224084";

Connection = "keep-alive";

"Content-Length" = 36178;

"Content-Type" = "image/gif";

Date = "Mon, 27 Jul 2015 12:57:35 GMT";

Server = "Microsoft-IIS/7.5";

"X-AspNet-Version" = "4.0.30319";

"X-Powered-By" = "ASP.NET";

Any idea why "cachedResponseForRequest" is not getting called?

Thanks!