Sounds like the USB adapter is not providing enough power. Most Amazon Echo devices require at least 1.0 or 1.5 Amps of power at 5 volts DC. Older phone chargers are only rated at 0.5 amps (sometimes written 500mA). So make sure your USB adapter is providing enough power.
Another thing, if you are using an extra long USB cable, say more than 6 ft, there will significant power loss due to the length of the cable. Avoid using long USB cables.