This can happen if your candle isn't burned long enough the very first time you burn it. This is unfortunately a common issue called "tunneling" that can happen if it is blown out too quickly the first time. Here is some helpful info from Apartment Therapy (and also a solution below it):
'If you want your candle to burn evenly and last as long as possible, you have to light it for about 3 to 4 hours the first time you burn it. “We say that wax has a muscle memory, and it’s going to burn in the same pattern every time,” says Heyen. “So if you light it for long enough the first time, that will ensure that your candle pools completely across all of the wax, setting it up to continue to do so with each subsequent burn.” Otherwise, tunneling, where the wick burns straight down the center of a candle without creating a full melt pool, can occur. Once your candle starts tunneling, you’re losing all that peripheral wax surface area for future burns.'
Thankfully though, there are a couple of options for fixing this problem! Your candle shouldn't be too far gone. You can either take a hair dryer on low to even out the surface, or turn your oven on to 175 degrees and pop it in for about 5 minutes. Just be really careful when getting it out. I would also suggest using tongs or something similar to pull the wick back into the middle if it leans to the side when the surface is melting.
