I had a similar situation happen to me earlier last year. I booked a flight with United Airlines for a trip in July, but ended up having to cancel it. I was also within the 24-hour cancellation period and entitled to a full refund.

However, when I checked my credit card statement, I noticed that the charge was still pending even though I had cancelled the flight. I reached out to United customer service and was told that the refund would be processed within 7 business days, which obviously didn’t happen. It took much longer than that for the charge to be refunded. It ended up taking almost a month for the charge to drop off my credit card statement and had to contact my credit card company to move things forward. I was frustrated by the delay and felt like I wasn't getting clear answers from United about what was causing the holdup.