Finding bugs is somewhat of an art. Every tester has different strategies and methods they use to test a site. These will come with practice.
In the meantime, if you're feeling stuck in a test or unable to find bugs, here are two suggestions:
Reproductions: We pay testers for reproducing bugs already submitted by other testers. Reproduction requests can be found on your tester dashboard or you can manually open other testers' reports to find the reproduction option. Follow the steps to reproduce the observed result and submit your findings.
Be a User: Pretend you are a user of the site. Progressing through a natural flow is sometimes the best way to find issues. For example, if it is an e-commerce site, try browsing items and searching keywords. Add something to your cart and begin the checkout flow. Putting yourself in the user's shoes is an excellent way to catch issues that may be harming the site.