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

Asked by for the 2002 Dodge Ram 1500
'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.