When someone makes a will it needs to be done of their own free will. If you suspect it was not and you lost out, you might consider filing a contest on the grounds that someone used undue influence to have them change the will in their favor.

Undue influence is a legal concept. It does not just mean you think that someone curried favor with the person who created the will by being nice to them. You need to show there was more to it than that and that it was somehow devious and intentional.

An imbalance of power

Typically you will need to show the relationship was unequal and that the person doing the influencing had some sort of power over the person making the will that they exercised to have them make the changes.

Examples could include a care home nurse whom an elderly person relies on for help with everyday tasks like bathing and eating. Or a family member who looks after and advises a young adult with severe learning difficulties.

Opportunity

Aside from the power imbalance you'll need to show that they had the opportunity to use their power. If the deceased spent just as much time with other nurses or family members then that would be difficult. 

The onus is on the person contesting the will to show there was undue influence

Even if you are sure there was undue influence, you still need to prove it. Courts know that people often have valid reasons to make will decisions that others might find unreasonable. Perhaps the elderly person in the care home just figured that their nurse has been far kinder to them than any family member ever was. Or that she or he needs the money far more than their own children who already have plenty.

If you believe you have lost out due to someone exercising undue influence over a loved one, consider legal help to examine your chances of a successful case.