Are you working with a real estate professional? An agent will be able to provide guidance regarding many issues, including fair offers that are based on solid comps.
If your approach has been to simply throw numbers at the seller without validating them with both accurate and current information, you will likely meet with continued defeat.
Since most buyers are not realistically aware of the real value of their homes, they need solid facts to bring them back to reality. Low offers today should be validated with supportive information.
Is your Buyer's Agent giving you adequate information and assistance in helping you structure your offers? Is your agent treating the Listing Agent and sellers with respect?
This may seem trivial but human beings tend to try harder to get the deal worked out when they perceive the other side to be congenial and pleasant.
Good luck in your quest;
May you find a happy home!