It sounds like you may have to get an attorney, and either file, or threaten to file, an action to determine the boundary. There are several things to consider in that regard.
First, how confident are you about the surveyor and his survey? Different surveyors tend to disagree, and sometimes they can both be "right," if they are dealing with a problematic survey, or problem areas. You should ask your surveyor how confident he or she is about their survey.
Secondly, even if the survey is accurate, there may be a claim of adverse possession and/or boundary by acquiescence, if your neighbors and sellers have treated a mistaken line as the boundary for a long period of time (generally 20+ years -- this varies considerably from state to state).
Boundary problems are best resolved, when possible, by negotiation. It sounds like you are trying this, but you may need to assert your full rights, in order for them to acknowledge the issue and agree to compromise.
I'd recommend you find a very competent and experienced property lawyer. Your surveyor may be a good source for a referral.
My advice, of course, is general and based on very little knowledge of your personal situation, and I am licensed to practice law only in Idaho, Utah and Wyoming. You should retain legal counsel and review you situation with him or her, if it has significant financial or other implications. <a href="http://www.haolaw.com" target="_blank">Visit www.haolaw.com!</a>
Boundary disputes are best resolved by cooperation between the parties. Sometimes it may be beneficial to use a mediator if the parties are willing. If the parties are still unable to come to an agreement then it is best to initiate a court proceeding to determine the boundaries. Adverse possession and boundary by practical location actions require the same criteria to be met. The period of time needed to establish the claim is 15 years in Minnesota. Minnesota Statutes Chapter 559. Adverse Claims to Real Estate will be informational regarding this situation.