Making changes to your domain name DNS settings can be tricky. Sometimes making the changes doesn't work as you expected, and there can be various reasons for this. The most common reasons are:
1) You have entered the settings incorrectly. This is the first thing to check.
2) Control of the DNS settings has been changed from your domain registrar, to a third party.
Regarding 2, if you have previously had a website and / or email for your domain, you might have given control of the DNS settings to a third party. You can test this by going to this link:
But replace 'exampledomain.com' with your own domain (don't include the www). The results on that page will give you a clue as to who controls your DNS settings. It might be your email provider or the company that made your previous website.
You can also check by logging into the control panel for your domain registrar, and checking that the NameServers are set to the defaults for your registrar. If not, then control of the DNS has been passed to a third party.
In the case that control is with a third party, you have 2 options.
Option 1: Change the nameservers back to the defaults.
This can take up to 48 hours, but might be as quick as 30 minutes. Once changed back, you can control the DNS settings from your domain registrar control panel. However, you need to be careful doing this because you might interupt your existing email service if you have one. Changing the NameServers will reset the email configuration records. To avoid this problem, you need to get your email records from your email provider, and add these to your domain registrar control panel. Then once you have done that you can reset the nameservers.
Option 2: Make the DNS changes to map your domain with the third party.
This is the easier option, but it might not always be desirable, or possible. If you are keeping your email with the third party, then go for this option. If you are no longer using the services of the third party, or you can't get hold of them, then you'll need to go with option 1.