Unit tests using Nocilla failing randomly

Current Setup : AFNetworking + Kiwi (for unit tests) + Nocilla (Stubbing network calls)

Issue Faced :

Some of the tests that use Nocilla to stub HTTP request fail randomly at times.

__block NSString *fetchedData;
[someData updateDataWithHandler:^(NSString *errorMessage, AFHTTPRequestOperation *operation) {
                        [[errorMessage should] equal:@"No response."];
                        fetchedData = (errorMessage) ? @"Failure" : @"Success";
                    }];

 [[expectFutureValue(fetchedData) shouldEventuallyBeforeTimingOutAfter(4)] equal: @"Failure"];`

At times when this method fails, the assertion says

expected subject to equal (NSString) "Failure", got ((null)) (null)

Any ideas into what might be causing these tests to fail randomly ?