Why dropdownlist binds limited data when we using JSON (from database)

Most of the time JSON max length exceeds (data) limit to occur this problem , try below coding in web.config to avoid these issues. Just increase the JSON length.

In the <configuration> section, add

<system.web.extensions>
    <scripting>
      <webServices>
        <jsonSerialization maxJsonLength="500000"></jsonSerialization>
      </webServices>
    </scripting>
  </system.web.extensions>