There should be many reasons. Check with few things:
If you have created a new website, and you are facing issues while indexing. You should wait for 2 to 3 months. And after that take any action and issues can be anything like:- meta robots tag, robots txt, crawling issues. For fast getting fast indexing on the search engines you should create a sitemap and add it on robots txt file, submit a sitemap on the search console, and create more backlinks.
create sitemap and add