The heater/AC blower seems to have a mind of it's own. Sometimes the air blows through both the center vents and the side vents and at other times only the side vents. Annoying to say the least.

Thanks for your replies.

 

there are several condition that your a/c unit will open the center vents.
1. I believe there are like 7 speeds for the fan. the center vents are close from 1 to 4th speed then the a/c unit opens the center vents.
2. if you set the temp to warm the center vents will never be open.
3. if you set the temp to cold the center vents will blow cold air no matter what speed.

these are the settings for auto. you can always manualy turn on off the center vents.

 

Do some searching on vacuum leaks in regard to the distribution flaps/vents.

 

Many thanks for your information and suggestion