Phone stops charging once it hits 40% capacity

I bought a used nexus 5x on craigslist and the seller sold it to me without a working charger. I didn't know this at the time but no big deal. I bought a USB to USB C cord and have plenty of power bricks lying around from my iPhone days.

I was able to reach 100% charge that day but, ever since it just gets hung up at 40% and it doesnt even say "Charging" while plugged in anymore. The battery icon at the top still shows that it is plugged in but when I check the status it doesnt say "charging on AC" like it once did.

I've only ever seen "charging" or "charging slowly" while its been plugged in. I ordered a different power brick that should be here within the next day but, i'm worried I'll come across the same problem there.

Anyone have any advice for me?

Thanks.