Subaru Air Vents

The air vents on my 1999 Subaru Legacy are only partially working. Right now air only comes through the middle set of vents (ie, not the windshield or floor).

Last winter the other vents worked occasionally. After keeping the slider at Windshield for a day or two all of a sudden the others worked fine for a couple weeks. I replicated this a few times but then the trick no longer worked. I also experienced a problem getting heat to come through the middle vents once the others stopped working. It was hit or miss. Often though, the heat would start working if I kept it on continually and drove on a highway. No idea why that worked!

I need to get my A/C recharged and am wondering if I’ll be able to get cold air through the middle vents at least-- ie, will I experience the same problem as with the heat?

Thanks for your input.

To clarify, I could always get air to come through the middle vents. It’s just that it wasn’t always heated.