Visualforce page action method does not get invoked in force.com site

I am using a method is page's "action" attribute. When I am testing this by login into salesforce, it's working fine. But when I used this page on Force.com sites, the action method didn't execute every-time. Sometime it works and some time don't work.

Can anyone suggest why it's behaving like this ?

VF Page:

<apex:page sidebar="false" showHeader="false" standardStylesheets="false" applyHTMLTag="false" applyBodyTag="false" controller="APTS_eCommerceController"   action="{!setPrereqs}" >

</apex>

Controller :

public void setPrereqs(){
 CustomObject obj = new CustomObject();
insert obj;
}

Attribution to: S.Sharma
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