Phone percentage

Probably because you aren’t leaving it on the charger long enough. But here are some other possibilities:

When you fast charge your iPhone it will stop at 80% to allow the battery to cool down. Some time later it will resume if you leave it plugged it, and charge more slowly, tapering the charge rate until it reaches 100% and charging stops. It can take another hour or two after reaching 80% to complete the charging.

Another possibility is if you have Optimized Battery Charging enabled. This is intended to be used with overnight charging, which is the absolute best way to improve the life of your battery long term. Enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. 

If you don’t charge overnight you should probably turn Optimized Charging off, because it will stop at 80% as it tries to figure out your usage pattern. 

Final thought: Are you using an Apple power source and Apple cable, or at least ones labeled as "Made for iPhone" (MFI)? If not, that might be your problem. Some power sources will shut down when charging stops, even temporarily, and won't resume until the phone is disconnected, then reconnected.