I am new to orchids but I'd guess that it is somehow related to your high altitude and lack of humidity. I've found that most orchids sold here commercially are grown in the foothills of Veracruz. Although, whenever a nursery plant dies here in Mérida people say it is because it was grown in the greenhouses around Mexico City and not adjusted to our climate.
Give them more light and more humidity, that's seems to be the usual problem.
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