iPad battery level doesn't increase when in use while charging
This could be normal if you have a lot of power being used (stuck process, bad iOS load, games or many background apps). If you put the iPad in airplane mode and power it off. Connect to power and then be sure to only do things that use little power like play a song or two or read iBooks. You’ll know which apps you run (games are notorious for taking more power than the charger can supply) and then know if even the lightest load is enough to prevent meaningful charge.

If you in fact have a device that under low load can’t charge, that’s not normal. This can be a symptom of three things:


The iPad battery is failing - when they age this will happen to 100% of all batteries (eventually). Some batteries will fail like this before their useful life is up, too. Ask apple to run battery diagnostics and you will get a reading on the health as well as the cost to have it serviced if you ask. This service is free for iPads 5 years old or less.
The charger or cable just isn’t sending enough current to fill up the gas tank while the car is being driven. Get or borrow a new cable and a new charger to test this.
The iPad charger and pins are failing - when you rule out the other items, this remains. Hardware service will fix this.