Greetins and welcome!
Usually, when we change our nameservers for our domain to new one (Cloudflare for example), it could take up to 24-48 hours to apply the changes due to the recent DNS change at the domain registrat and the regular process of the DNS propagation time frame around the World.
Furthermore, maybe some of your visitors also have an small issue like the local DNS cache or their ISP provider DNS cache which could be slightly different.
There are some online tools for the DNS check where you can check if it’s the correct value for the needed DNS hostnames like A or NS type.
You could try to enter your domain name (without www prefix) at the input field on the given link from below, and select from the dropdown the “NS” first and click on the button to check, thereafter you could also repeat the same but select the “A” from dropdown and re-check.
All the results should have the same value if DNS propagation went successfull.
Or, kindly you can post your domain name here so we could also check from our Web browsers and see what do we see, just in case.
Furthermore, you could try to flush the DNS cache for your domain by using below online tools (by entering your non-www domain in the field and selecting the desired A, NS, etc.):