There are some cases, why the workflow was not started during item creation,
1. If you create an item using System Account, the workflow will not funtion properly or that couldn’t start.
2. The workflow start option was not enabled to “Start workflow automatically when an item was created”. Ensure this option was checked.
3. If you create and publish a workflow without enabling the Start option(Start workflow automatically when an item was created). And after some time you came and enable that start option & saves without Publishing also cause a problem.
4. Or remove the “Allow this workflow to be manually started” start option and Initiation workflow. Enable “Start workflow automatically when an item was created” option and Publish the workflow.
check the following link has the soltuoions for SharePoint Designer workflows, http://office.microsoft.com/en-us/sharepoint-designer-help/troubleshoot-workflow-errors-HA010237912.aspx#BM2
If the above solutions couldn’t solve the problem, the you have to create new workflow.