Vacuum Exhaust Valve

I have a 2001 330i and the exhaust flap is staying open. I have read a lot about these flaps and I know a lot of people will tell me to forget about it and leave it open but for several reasons I want it to work. I have removed the solenoid from its housing in the boot and I've pulled the vacuum pipes off it. I put my finger over the tube, I expected it to suck quite hard on my finger but there was virtually no vacuum, there was a little bit but nothing like I expected and I think this is the reason for the none closing flap. I have sucked on the tube that goes to the flap and had to suck quite hard but the flap did close. There is nowhere near enough suction from the pipe to close the flap. I'm wondering if anyone has any idea what might be going on as I'm struggling to follow the pipe forward further than what I can see in the small corner of the boot.