Apple has taken action and have launched a replacement program for USB-C cables that come bundled with 2015 Retina Macbook units. Faulty cables are to blame, and you can find the image at the bottom of this article to find out if yours is faulty. Apple have said bad cables would result in the MacBook not charging, or charging unreliably when connected to a power adapter.
Apple has it has affected a "limited number" of USB-C cables. Apple did not offer an estimate as to how many affected units reached customer hands, but a review of the accessory posted to the Apple Online Store last September appears to describe the problem:
While the company did not offer detail on the apparent design flaw, it did specify a method by which old cable versions susceptible to failure can be identified. Apple says redesigned cables include a serial number next to printed text reading, "Designed by Apple in California. Assembled in China." while original versions do not.This cable has gone from kinda working (getting my macbook to charge after 20 secs of trying) to having to fiddle with it for more than 5 minutes before it works. As of now, I have to manually hold the charge cable into the port as far as it will go before it registers and begins to charge. The moment I let go it stops charging again. Then, randomly, it will suddenly resume charging as usual. This is incredibly frustrating and time wasting.
The replacement process can be initiated at an Apple Store with a Genius Bar appointment, at an authorized reseller or online via Apple Support channels. The program runs through June 8, 2018 so you have a couple of years to get your cables swapped out.
The USB-C charge cable program is effective until June 8, 2018 so exchange them now!