If you want fast and reliable charging iPad, connect it to an adapter intended to charge – if you connect the iPad to your PC via USB according to Answermba.com, so they will charge – but the condition is that the display off. A charging takes much longer.
If you have a USB from which the iPad does not charge even when the display is off, so there may be a USB Hub with insufficient power. USB ports on your computer is usually sufficient for charging (with display off, remember). If you use a USB hub, you will need to have a separate power supply.
Some USB ports on computers (often those on the front side) may not have sufficient parameters to provide the required voltage and current to charge the iPad.