I would suggest eliminating the simplest causes first, no insult intended;
1. Did you just buy Polarized Sunglasses? HUD is not visible with those (at least those that I've used)!
2. Inadvertent adjustment of HUD position too high or too low.
3. As suggested earlier, HUD dimmer adjustment too dim?
4. Dimmer selector switch in-between positions of Dash or HUD select, or switch is bad.
I went with my wife to test drive an 04 before we decided to buy one. She went with the salesman first and then I drove it second. On the way back home we're talking about the car and she asks me what I thought of the HUD. I wasn't even thinking about it at the time as I was smashing the pedal to the floor on the on ramp to the interstate.. but I didn't see anything. She was like.. what are you talking about you didn't see anything?? I thought maybe I hadn't "turned it on". She proceeded to inform me it automatically comes on... I was beginning to become perplexed at this point.. I thought, there's no way I just looked through it and didn't ever see it. It drove me nuts thinking about why I didn't see the HUD and she did.
A week later I'm in Miami taking my new car for a test drive. It's raining and she didn't want to drive it so I drove it first this time. Get in the car, wham.. HUD all in my face. Sweet! Now, I'm even more confused why I didn't see it the first time..
A couple days later I'm over at a buddy's house and I'm going to take him for a spin. Jump in the car, start her up... HUD all in my face. Alright, we're ready to roll, glasses on... wham, HUD disappears.
I was elated at this point. It just all clicked together..
On the first test drive it was a bright sunny day so I got in the car with the sunglasses already on. Never saw the HUD. Went I bought the car and took it for a test drive, it was raining.. no sunglasses.