It could be a number of things. Do you plan on doing the testing and repairing or are you just looking for some possible causes?--mbsastronomy.com
reply to mrsnich Are you low on coolant?Are there any warning lights illuminated or flashing?--The talented hawk speaks French.
reply to mrsnich If the car does not reach a certain temperature, the overdrive will not engage. Another possibility is a faulty solenoid.