18w Apple Fast Charger not charging my 11 Pro Max

I recently purchased a new 18w Fast Charger for my iPhone 11 Pro Max seeing as I bought the phone from an outside provider and it didn’t come with its own. This charger will charge all my other Apple devices, including other iPhones (previous models), but won’t consistently charge my Pro Max. I’ve tried other chargers and they work perfectly well, so I’m assuming it has to do with the Fast Charging feature on my iPhone. I’ve gotten it to charge my phone twice after minutes of fiddling around with it, but haven’t come up with a concrete resolution. Has anyone else experienced this or have any knowledge on this issue? If so, if you could please share.