I am not new to the hobby, but I am new to DCC. I recently built my first layout in my house (I previously ran on my dad's DC layout). My layout uses exclusively EZ Track, primarily the silver roadbed with some black mixed in, with EZ Command Plus. I successfully assigned channels to turnouts and a DCC locomotive (Athearn). However, when I turn the throttle to run that locomotive, I have to continuously turn the throttle--most of the time; occasionally, it will run for a few seconds without constant turning of the throttle.

I am fairly certain this is not a bad EZ Command box because I actually returned the first one I got as defective based on this issue. This issue also arises on my secondary test/programming track, so I thought it was the box. However, my newest one also has the same issue, so I doubt I received two EZ Command boxes with identical defects. Any ideas what my problem could be?

A video of the problem:
If I need more power...then I'll need help because I bought a 5-amp booster, but I cannot decipher how to actually connect the booster to the tracks. Some threads indicate that I should be able to splice the wires that came with the booster to an EZ Track clip, but I have no idea how to do that, and I couldn't find a thread explaining how to actually do it.