Those are some great suggestions.

I have the bagless kind and I have tried cleaning it all out to no
avail.
I'm going to try the suction right at the unit then I'm gonna try the
reverse suction method John mentioned.
If that doesn't do it, i'll have to look into getting somebody in to
try their magic.

One thing though: sometimes when I plug the hose into the "tap", it
takes a while for the vacuum to come on, I usually have to wriggle the
hose around so that the metal band latches properly to the "trap". Do
you think this might be part of the problem? maybe not enough of a
connection?

I'll let you guys know how it goes and thanks again.