Certificates requested through Amazon ACM are stuck in "Pending validation"
Solution 1:
AWS will periodically check the DNS entries. In the past I've done exactly as you have, and found checking back, after 5-10 minutes is enough.
If after re-checking the console, and you still aren't validating properly, check using an external tool if the records are public.
There are a number of reasons why DNS changes could take a while to make it to AWS, but a few hours is longer than I would expect. That implies the real result is a misconfiguration.