In the 1980's I bought a Vax 4000 wet/dry vac. It would pick up anything that would physically fit down the tube – swarf, stones, water, you name it, and down it went. At some point the tube, or maybe the handle, can't remember which broke so I bought a replacement from our local hardware shop. From that moment on, bits of "stuff" would wedge, usually where the hose entered the handle, and eventually block the tube. The cause was an un-smooth transition from the handle to the tube. What I did was to remove the locking ring and rely on the Vax's suction power to keep the two parts together. If it blocked up, it was then a simple matter to pull the hose out of the handle, clear the blockage, and replace.
You don't actually need smooth tube if you do what I did, and still do today.
Peter G. Shaw