Flex should always be pulled snug to help with airflow. Tape is sold in the furnace pipe section in hardware stores. All leaks should be sealed,supply & return.
Current Code suggests FRP(Fiber Reinforced) thats what I would use.
It Could be loose but still be a good connection, just have to look closer.
Check for kinks ,leaks....Distance.. from unit on low flow, maybe could put larger duct in to increase flow.
Theyre are many closer If you still have problems in the spring let me know.
Any decent HVA/C tech should be able to check the airflow at each outlet with an air flow meter. Corner rooms with extra windows should be given a tad more air flow than the rooms near the center of the house. Unfortunately, it usually turns out to be just the opposite. This is due to backpressure in the system. The blower in the furnace is cabable of only putting out a certain amount of pressure. Ducts should be sized for the specific amount of air they should supply to each room. Years ago I worked in this business, but sad to say, I have forgotten all of the rules of thumb that I used. For instance a 8" duct will carry 4 times the air of a 4" duct.