Low serotonin levels are frequently linked to anxiety. Most of the popular antidepressants and anxiety drugs are Selective Serotonin Reuptake Inhibitors (SSRIs), meaning they stop the breakdown of serotonin in the synapse, increasing the activation of serotonin. This is great for someone whose anxiety is potentially caused by low serotonin levels so they're typically the first drug that someone is prescribed. But this study shows that people experiencing anxiety may have HIGH serotonin levels, so giving them an SSRI would only make the issue worse.
There are atypical (non-SSRI) anxiety and antidepressant drugs, the most popular being Wellbutrin. So there are options for people who don't respond to SSRIs.