Lightning Dock Lower Power Output?

I've noticed that when I plug my iphone 7 plus into the lightning dock, rather than plugging it directly into the lightning cable, the power output is drastically different. Sometimes the output is so low my iPhone 7 Plus won't even charge. I have multiple of these and the result is the same on all of them no matter what the source of power. Are there any known issues with this lightning dock is known to have lower power output?