*ngIf condition is not fired/doesn't update template in ngOnChanges

I have a child a component that does not fire the condition for *ngIf

child component:

 export class child {
    @Input() user;
    @Input() list;

    listLength: number;
    showBtn: boolean = false;

 constructor(){}

 ngOnChanges(changes: SimpleChanges){

     this.userId = this.user.id;
     this.userList = this.list;
     this.listLength = this.userList.length;   // this updates everytime when changes happen on another component thats pushed from this component.

     if (this.listLength > 3){    // this doesn't fire until I refresh the page
        this.showBtn = true;
     }