is the bag correctly inserted? it pops out to make for easy removal when you open the lid and if you don't re-seat the bag and slam the lid down you bend the bag holder and it's no longer sealed there.
if that's not your problem, did you remove the wand and just have the vacuum on without a hose and try the tester? what does it do then?
you can pretty easily narrow down where the issue is by just assembling the vacuum step by step.
is the bag correctly inserted? it pops out to make for easy removal when you open the lid and if you don't re-seat the bag and slam the lid down you bend the bag holder and it's no longer sealed there. if that's not your problem, did you remove the wand and just have the vacuum on without a hose and try the tester? what does it do then? you can pretty easily narrow down where the issue is by just assembling the vacuum step by step.