It's true, hot air rises. It also radiates. The flooring acts as a conduit to transfer heat through it, walls, ceilings and floors. When you insulated the basement I assume you insulated the basement ceiling. If so, you slowed the transfer of radiant energy through the structure toward the basement. Warm always travels toward cold.
Do you have any ducts that you can open in the basement? Do you have access to any HVAC trunk lines where you could install a couple?