iPad Air 4 Battery

Every time I will charge my iPad, it stops at 80% and I don't really know why. I tried to restart it every time it will get to 83 or 85, and I will plug it again but there's still no progress. There are times that it will reach 100% but right now it's still stuck on 80% whenever I tried to charge it. I don't know if it's because of the software update or the battery itself.