There are a number of reasons a Will can become invalid and we recommend you always seek professional legal advice on your Will and Estate matters.  The following are just some instances that would make the Will not valid:
You make another Will,
You lacked the mental capacity to make the Will,
Someone else used undue influence or pressure to force you to make the Will,
The Will isn’t signed and dated by you and two adult witnesses,
Your personal circumstances change, such as a marriage or divorce,
It includes assets which are not yours to bequeath.