In addition to the IWE solenoid, you could have a vacuum leak, or your vacuum pot could be defective, or your check valve may be defective. In my '07, the vacuum line that runs across the front crossmember was worn through. A quick splice and my system was working again. A simple hand vacuum pump with a gauge can let you test each component:
1. Test the vacuum pot (reservoir) by connecting it to the hand pump. Pump it down to 10-15 inches and see if it hold for more than 30 to 60 seconds. If not, it's defective. If it holds vacuum, it's good.
2. Test the check valve by connecting each side to the hand pump, one side at a time. One side should hold vacuum. The other side should free-flow.
3. Test the solenoid by connecting the supply inlet to the vacuum pump and pump it down to 10-15 inches. It should hold for at least 60 seconds. Then, turn on the car and make sure the selector switch is in 4H (or 4L) but not 4A. The vacuum should drop as the solenoid applies the hand pump vacuum to the IWEs. You should be able to pump up more vacuum and the IWEs should engage and hold the vacuum with zero leakage or a very slow leakage. If you can't hold a vacuum with this way, then the vacuum lines or the IWE's are bad.
4. Hook up the pump to each IWE and pump it down. It should hold steady-ish for 30 to 60 seconds at least. If it won't hold a vacuum then the IWE is bad.