From the 2013 State Department report:
Countries with Enforcement Concerns:
Mexico: Convention return order not enforced
Unresolved Return Applications
As of December 31, 2012, the U.S. Central Authority (USCA) had 112 applications for return that remained open and active for more than 18 months after the date of filing with the relevant foreign central authority in the 18 countries listed below. The following section describes each unresolved case and the actions taken by the USCA to resolve them. The actions taken by other authorities are stated below, as reported to the USCA by the relevant entity.
Follow the below link to the full text of the 2013 Report to read the Case Summaries involving Mexico (there are over 60 full case summaries of unresolved return application from Mexicos in the 2013 report):