How to Get Cold Air From One Room to Another?

There are a few different reasons why one room may be colder than another:

Some rooms are on the side of the house that gets less sun exposure. Along with being darker, these are often cooler because the heat of sun rays is not influencing them. 

If an AC unit is installed in the room, it is being fed with cold air and will have a lower temperature than the rooms around it.

The natural separation of warm and cool air can also result in lower rooms being colder than upper floors as cold air sinks and warm air rises.