Parental alienation is in essence where one parent manipulates a child into fear, disrespect, hostility and/or rejection of the other parent.
One of the biggest problems in situations of parental alienation is that the alienating parent is often the parent the child lives and spends most of their time with. It can be very difficult to dilute or control the influence of a primary carer on a child.
As any alienated parent will know, often their child adopts the mentality of the alienating parent and so, on the face of it, is similarly against contact with the alienated parent. This can be difficult to see through it is vital to involve appropriately qualified and specialist expertise early on.