Hello All -
I purchased Frontier flight from ATL, GA to CUN, Mexico roundtrip refundable ticket and was denied entry at CUN airport. There was no written/proper explanation provided by the officials for denying entry at the airport except that I was not having valid documentation to enter their country and they mentioned that airline officials should have checked for the documentation and not have allowed us(4 family members) to board the flight. Immigration officials did not provide it in writing though.

Frontier got me on the next flight back into to STL, Missouri instead of the ATL, GA airport as they did not have flights until the next date. I did not see Frontier agents at the STL airport and took a one way rental to get back home. I reached out to Frontier Support for ticekt amount refund and they are denying any responsibility or fault of theirs. I am not sure what options should I pursue to get my refund. Please help/suggest.

It sounds to me like you used the return protion of your ticket to travel to STL instead of ATL. Had the flight back to Atlanta already departed by the time Mexican immigtation denied you entry? If you had only bought a one-way ticket on Frontier (and a return ticket on a different carrier, since you need proof of a return flight) they would have had to transport you out of Mexico at their cost.

Do you have an American passport or US legal permanent residence (green card) or is your immigration situation more complicated? It is pretty rare to be denied entry to Mexico (and especially in Cancun). This is likely an issue Frontier should have caught if you have a passport from a country which requires a visa to enter Mexico. I'm not terribly surprised Frontier didn't catch this but I am surprised they let you travel to CUN. If there is any doubt about admissibility, the carrier usually denies boarding before departure to the destination country so the doubt can be clarified (and so they don't get fined by the destination country for transporting an inadmissible person). At the end of the day, it's your responsibility to possess and present the required documentation to enter your destination. I don't think a refund or compensation is warranted in this situation. It's simply unfortunate.

Roundtrip with Frontier on same itinerary. There was no flight available for ATL when we landed. Am a legal resident in US. Have travelled to Cozumel, MX on US Visa and am now allowed to travel on AP.

I agree it was unfortunate but Frontier also was negligent in this case. Hence wanted to find out if there are any process or obligations that air carriers should follow to avoid such situations?

Very strange that Mexico would deny you entry based on the facts above. US citizens and LPRs are basically admitted automatically to Mexico for up to 180 days. It would be good (for you) to get to the bottom of why you were denied entry if you would like to return. Without knowing why you were denied entry it's really not possible to comment further other than to say it's your responsibility to comply with the immigration rules of your destination country (and transit countries, if any).

Frontier should verify you have the correct documents before boarding you on an international flight. This would normally inclue a passport and visa, if required. Their punishment if they fail in this duty is a fine from the destination government and they have to remove you from the country on the next available flight. However, not all denials of entry can be foreseen. Some passengers have all of the correct documents when the airline checks them but are denied entry for reasons the airline can't forsee (e.g. a previous criminal conviction). In these cases the airline is not fined but they still have to remove you from the country. No additional compensation is due.