You may be used to using a computer to charge your phone, but charging an iPad takes more time. If you have to use a PC to charge, this may cause the iPad to not charge.
The iPad charger is for iPad, the smaller iPhone adapter will provide half the power. If you have an iPad Pro, the iPhone will take longer to fully charge.
Want to charge quickly? Use the iPad's AC charger to charge your iPhone faster
Reboot the iPad. If a software glitch is making the iPad charge slowly or not at all, a simple reboot may be enough to fix it.
Test the charging cable and adapter. If the iPad doesn't charge through the electrical outlet, you may have a problem with the cable or the adapter.
Troubleshoot the cable by connecting the iPad to your computer. If you see the lightning bolt on the battery meter, you know the cable is working. If the computer doesn't react when you plug in the iPad, try another cable to see if that fixes the problem. If it does, toss the first cable.
Similarly, try switching power adapters if you have more than one, or borrow one from a friend. If the adapter doesn't work, but another one does, you'll need to buy a new adapter.
Clean the iPad's charging port, the most common problem is dust/lint/getting into the port
