iPad Pro 12.9 stops charging after a minute or two

If I unplug the charging cable from the iPad Pro and plug it back in, then the charging resumes, but will stop again after a few minutes. I am unable to determine what causes the charging to stop. If the iPad is locked, then it usually charges to 100% without stopping. As a workaround, I have to plug the charger in when the battery gets very low, and leave it locked while it charges. I unlock it after it gets to 100%. I have tried multiple chargers and charging cables, and the issue occurs on all of them.