Keychain is being reset itself in ios

Question

I am using keychain class for storing some value in keychain . By using these keychain default methods

KeychainItemWrapper *item = [[KeychainItemWrapper alloc] initWithIdentifier:@"identifier" accessGroup:nil];

 [item setObject:@"some_value" forKey:(id)kSecAttrService];
[item setObject:@"Some_value" forKey:(id)kSecValueData];

values is being saved successfully in keychain When I close app and start it again and initialize keychain object

then this  condition in this function of keychain

- (id)initWithIdentifier: (NSString *)identifier accessGroup:(NSString *) accessGroup`

is being true, cause of that my key is being reset and returns no value

    if (! SecItemCopyMatching((CFDictionaryRef)tempQuery, (CFTypeRef *)&outDictionary) == noErr)
    {
        // Stick these default values into keychain item if nothing found.
        [self resetKeychainItem];

        // Add the generic attribute and the keychain access group.
        [keychainItemData setObject:identifier forKey:(id)kSecAttrGeneric];
        if (accessGroup != nil)
        {

            // Ignore the access group if running on the iPhone simulator.
            // 
            // Apps that are built for the simulator aren't signed, so there's no keychain access group
            // for the simulator to check. This means that all apps can see all keychain items when run
            // on the simulator.
            //
            // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
            // simulator will return -25243 (errSecNoAccessForItem).

            [keychainItemData setObject:accessGroup forKey:(id)kSecAttrAccessGroup];

        }
    }

cause of that these methods return nil

[item objectForKey:(id)kSecAttrService];
[item objectForKey:(id)kSecValueData];

I don't understand , when value are saved first time then why this condition is being true.

can anybody help me

thankss