Once you have more than one Echo device, you need to begin noticing which device is being controlled by the App. Foe example, some notable sections of the app are Settings, Now Playing, and Music and Books. Once in either section, you will see a sub menu for (in your case, an Echo label, and in the dropdown menu, your Dot.) So, if you want to play music on your Dot, but the Echo label is shown, you must switch to the Dot, and THEN make you music selection, otherwise, the music will play on the Echo. The "gotcha" to avoid caused by having more than one Echo. I have at least 5 devices now to pay attention to!
This could also be the case if you want the Dot to control lights in the apt, and not the house lights controlled by the Echo. Someone with light controllers can verify this if necessary.