Dewalt 20V Trimmer-StartsThenStops- Help

Greetings.
RE: Dewalt DCST920 20V Grass Trimmer
We purchased this trimmer last summer, very pleased with performance & battery life.  Experienced no issues last summer.  After 5 min use this spring, unit stopped working. Both batteries fully charged.  Starts then dies within 2 seconds, rotates about 3 times.  We tried neighbor's Dewalt 20V battery, same result. 
 
Googled problems with this specific trimmer.  Many who have experienced this start then stop, replaced the bumper head with a Husqvarna bumper.  Part arrived, installed & our hopes for this to resolve problem fizzled.  We turned it on, head rotated few times & in couple seconds stalled.
 
What is the problem? How to Fix? Any input would be  greatly appreciated.
Thank you in advance.
~ Gem