UserInfo() function not available for infopath form in insert function option

I'm migrating the WSS 3.0 to Sharepoint 2013.In my migration i have 3 infopath form libraries.WSS 3.0 uses 3rd party webservice to get the user details.My requirement was to get the user name in the form without need of the 3rd party webservice. I was able to get the Current user value using the UserInfo Function for two forms.For One Infopath form I could not find the see the UserInfo() function in the Insert Function option.I have been stuck for two days.It will be really helpful if some one shares the root cause or workaround get me out of this. Thanks in advance.

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Cheers, Arul