Fields on forms are not recognized as CRM controls after import a solution from CRM 2011 to CRM 2013

I am having problems with the fields on forms of CRM Dynamics 2013, when I export a solution from CRM Dynamic 2011 and import inside CRM Dynamics 2013, some of my fields change to an html plain text and I cannot read the information of the same with XRM functions (Xrm.Page.getControl). Why can be this?.