Most often, chargers will appear unavailable in the EVgo app if it is under maintenance or reserved. Any unavailable charger will prevent you from starting a charge, even if using the credit card reader.
Some chargers may also appear busy, even if you're able to park there - while most chargers only have one dedicated parking spot, you may not be able to use that charger if it's already in use. Check for the Simultaneous EV Charging sign if another vehicle is already plugged in with another cable in use.
You can learn more the different station symbols in our Guide to the EVgo Map icons