1991 lincoln town car

My 91 i replace the heater core about a month ago the heat blows hot but it only works when i put it on high and it doesnt blow very hard out the vents either. Can somebody help with a responce thankyou

Assuming everything was assembled correctly :(

Sounds like two separate problems.. the air flow would be plenum doors not positioning correctly. The 'only on high' is the blower speed controller or wiring not connected properly.