Some Qi-based wireless chargers don't appear to be working with the iPhone 12 models, perhaps due to the magnets inside of the new devices, alignment issues, or other internal changes to wireless charging protocols.
Apple's iPhone models have been using Qi wireless charging since the launch of the iPhone X and iPhone 8 in 2017, so many people have existing wireless chargers on hand from prior iPhone generations. Unfortunately, some of those wireless chargers do not appear to be working properly with the iPhone 12 at this time.
There are complaints about non-functional wireless chargers from multiple new iPhone 12 owners on both Reddit and the MacRumors forums. Affected chargers work with older iPhones, but will not charge the iPhone 12 models, which is something to be aware of if you have a new iPhone 12 or are planning a purchase.
The MacRumors forums has a list of chargers that don't appear to be working with the iPhone 12 such as the Zens Liberty Wireless Charger, the Mophie Charge Stream Pad+, the Nomad Base Station Stand, and the Mophie 3-in-1 wireless charging stand. There are also complaints about some Anker wireless chargers.
Nomad's Base Station Pro with Aira wireless charging technology was previously on the list, but just this week Nomad released a firmware update that should address the problem. Unfortunately, most wireless chargers can't get firmware updates like the Base Station Pro, and it's not clear if there's a software fix on the way from Apple.
Some users have had luck getting a wireless charger to work after restarting a device or attempting to change the positioning of the phone on the charger, but in most cases, the affected wireless chargers simply don't charge the iPhone 12 models. This is a problem that seems to be limited to a small number of chargers and most appear to be working.
Update 10:15 a.m.: Apple has just released iOS 14.2 which includes a mention of a fix for an issue in which "devices could be prevented from charging wirelessly." Presumably, this should fix many of the previous issues, while other tweaks by charger vendors such as Nomad's Base Station Pro firmware update may also help. Not all chargers are able to have their firmware updated by users, however.