There could be many reasons for that. It may be that the quality of your site is poor, or it may simply be that you are in a high competition niche, and there are many good-quality sites all competing for the rankings.
If you are new to SEO, I suggest you start with Google’s “Search Engine Optimization Starter Guide“. Follow that carefully and ensure you’ve done everything right.
We have many threads here asking about increasing ranking, so I suggest you look around the #marketing section and read a few of those. For example:
https://www.sitepoint.com/community/t/increasing-traffic-and-ranking/223568/14