I have a Grizzly 3hp dust collector that I've had for years. Been working just fine. My "shop" is my garage so I used 4" PVC pipe to run overheads and drops to the various machines, etc. Did all sorts of things like putting sealing compound around the lid of the trash can used as a separator, sealed all the gates as best as possible, PVC joints, etc. Always felt that the suction was a tad low but I lived with it.
Last night I decided to change the way I had the original routing from the unit to the separator, so I had to remove the main Y joint at the unit - the DOH! moment came when I saw part of a plastic bag partially covering the main opening. It had been there for a long time because when I removed it, the resulting suction was noticeably different than it has been in years. Not enough to completely block the flow but enough to diminish it.
Just don't know why I hadn't thought of checking that junction first.
life goes on.....
Vic