Hi, it could be lots of things.
Firstly, if you are not adding content to your website regularly and your competitors are then Google may favor them.
Secondly, if you have not been building backlinks on a regular basis and they have then Google may also favor them.
If your onpage SEO is not up to scratch then Google may penalize you.
If there are duplicate content issues or site structure issues then Google may also penalize you.
My advice would be to run your site through a number of different SEO checkers and try to get a broad idea of where the issues may lie. You should also check your backlinks to determine whether your competitors are using negative SEO to push you down the rankings.