In Nondestructive Record mode, you can track layer upon layer, but only the topmost (first) track will take preference on playback, until you trim it to clear the way for the 2nd layer, and so on. This is how PT assures audio playback of a track. In order to hear the the 2nd layer, for example, you will have to use the Trimmer tool to clear a part of the 1st layer, only then will the 2nd layer take precedence. Same goes for hearing the other recorded layers, you have to "clear a way" for them to be heard.