When the list returned by the Web Service exceeds the max jsonSerialization limit of 102400 (string length), the AutoCompleteExtender
fails silently. A way to "fix" this (although is not practical) is to increase the value in the web.config as so:
Make sure you have this section in your sectionGroup section. If you don't, add it.
<sectionGroup name="system.web.extensions" type="System.Web.Extensions">
<sectionGroup name="scripting" type="System.Web.Extensions">
<sectionGroup name="webServices" type="System.Web.Extensions">
<section name="jsonSerialization" type="System.Web.Extensions"/>
</sectionGroup>
</sectionGroup>
</sectionGroup>
And then set the value:
<configuration>
<system.web.extensions>
<scripting>
<webServices>
<jsonSerialization maxJsonLength="50000000"/>
</webServices>
</scripting>
</system.web.extensions>
</configuration>