It's usually pretty easy to find answers to questions on the internet. Type in "what is the state flag of California" and you'll get a picture of it, even if you can't spell California. So why can't I find someone's death certificate by typing their name into ancestry.com?
The record you're looking for might not be on the site. It seems obvious, but I'm still surprised every time it happens to me.
Sites like ancestry.com can only search records in their own databases. They build their databases by asking for records from the agencies who own the records (often governments and churches). The record-holders can say no. And then the record doesn't end up on the website. Simple.
Or if they say yes, it can take a really long time to become available. Search engines can't search the text in the original records because they are pictures, so someone has to type every individual record. It takes forever.
When the American government released the 1940 census, millions of people helped type it out (they indexed it) and it still took several months to finish. You couldn't find anybody on the 1940 United States census until the government released it to the public and it was indexed.
Check out which collections each site has available to see which records you are searching. And keep checking back because they're adding new collections all the time.