DNS can be a tricky thing! One of the main problems with troubleshooting your DNS set up is that DNS is a distributed system, there are thousands of DNS servers around the world, and changes to DNS records take time to propagate to all of these servers.
A really useful tool for checking your DNS config is intoDNS. You just type in your domain name and it will run some tests and highlight any potential issues it finds. It uses the authoritative DNS servers to try and rule out any confusion caused by DNS propagation times.
Check it out here – intoDNS