It sounds as though you are new to the product, which is great and I welcome you to the community. Please know that labeling in ArcMap is basically its own course and takes a really long time to master. There are so many options, and so many combinations of options, that it could be difficult to assist you without further information, screen shots, and so forth.
By design, ArcMap does not normally show labels all on top of each other, so it has to selectively pick which ones to display and which ones to hide. There are ways to tweak this, but by default, it may seem random to you. Let us see your map at different scales before we suggest ways to configure the settings.
