Enemy won't move until I tab out?

I have never experienced this before but I have a simple movement system for an enemy character that gets executed when the player enters the enemies pawn sensing zone however when I executed the enemy remains where it is and doesn’t move. The odd thing is that if I tab out whilst running the game and then tab back into UE4 again the enemy is right infront of me and if I hold down Alt+Tab then I can see the enemy moving towards my player in the preview window. Any ideas on what I should do?

The enemy movement script looks like this:

FVector Player = GetWorld()->GetFirstPlayerController()->GetPawn()->GetActorLocation();
	FVector EnemyLocation = this->GetActorLocation();
	FVector MovementVector = Player - EnemyLocation;

	PlayerDistance = MovementVector;
	AEnemyAI::AddMovementInput(MovementVector, 1.f);