when i turn on the heat,i get warm air from the dash but the air at floor level, on 2002 Dodge Ram 1500

i recently had a new heater core installed at the dealer about a month ago,this problem just occured yesterday,what is the likely causes

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1 answer
'but the air at floor level,'....what? could you complete the question? And would it still be under warranty?More specific and complete details of problem will help get a better answer.