You are saying that you have CHECKED the 'Zoom Lock' button on map screen (2nd buttom from left at the bottom panel). With "checked", do you mean it is selected, i.e. yellow? It must be unchecked, i.e. black if you want to zoom normally. If it is yellow, you pixel zoom (which is not what you want to do in this case).
Also, Setting/Map/"Double sized resolution" should be unchecked.