From the 2014 State Department report:
Countries with Enforcement Concerns:
Below is a list of countries that are parties to the Convention in which left-behind parents in the United States have not been able to secure prompt enforcement of a court's final return or access order during the reporting period because of the absence of effective enforcement mechanisms.
Mexico: Convention return order not enforced
Unresolved Return Applications
As of December 31, 2013, the U.S. Central Authority (USCA) had 111 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 countries listed below. The following section describes each unresolved case and the actions taken by the USCA to resolve. The actions taken by other authorities are also stated below, as reported to the USCA by the relevant entity.
Follow the below link to the full text of the 2014 Report to read the Case Summaries involving Mexico (there are over 60 full case summaries of unresolved return application from Mexico in the 2014 report):