ArcMap 10.1 - Can't add grid references on the map

I am trying to add grid references from an excel spread sheet into ArcMap and my problem is that it doesn't display the data (grid reference points). This only happens with some of my excel tables but not with others. I have already displayed data in the same map and in the same format but I don't know why is not adding this particular one. Does anyone know why this happen? Many thanks in advance.