Why my left mouse button not fire the OnMouseButtonDown/Up event?

Hello.Here is my question.

I created the slate program and I implemented the “Onmousebuttondown” event,the Left mouse button can’t trigger the event, but the right mouse button can. This makes me feel strange.

I didn’t disable the left mouse button in C++ (in fact I don’t know how to). In the Playcontroller Blueprint, the left mouse button can be responded to. I’m confused.
Any help would be greatly appreciated.

Here is my code,it’s very easy.I just want to see if it can be fired.Only the right mouse button fire this event.

FReply SGrid::OnMouseButtonUp(const FGeometry& MyGeometry, const FPointerEvent& MouseEvent)
{
	if (MouseEvent.GetEffectingButton() == EKeys::RightMouseButton || MouseEvent.IsTouchEvent())
	{
		UE_LOG(LogTemp, Log, TEXT("OnMouseRightButtonUp handled"));
	}
	if (MouseEvent.GetEffectingButton() == EKeys::LeftMouseButton || MouseEvent.IsTouchEvent())
	{
		UE_LOG(LogTemp, Log, TEXT("OnMouseleftButtonUp handled"));

	}
		
	return FReply::Handled();

}