Well when they deny you upfront payment are they giving you a reason? Whats the length of your articles in general? Usually the lowest I will go is 400 words per article.
What kind of topics are you writing about? Usually I only submit topics like Health, technology, sports, investing. These all seem to do well while other just general opinion articles never get accepted.
Are you sure you are selecting the right things when submiting your article? Make sure you are selecting upfront payment and give them the sole rights to the article.