If you directly use the following version of the post method, then each request is separate and there will be no "previous one is missing" situation.

- (NSURLSessionDataTask *)POST:(NSString *)URLString
                    parameters:(id)parameters
                       success:(void (^)(NSURLSessionDataTask *task, id responseObject))success
                       failure:(void (^)(NSURLSessionDataTask *task, NSError *error))failure
{
    return [self POST:URLString parameters:parameters progress:nil success:success failure:failure];
}

Do you have the relevant cancel call?