There is no problem with air getting sucked in. The o-rings and gaskets are properly lubed and mounted. The pump sounds smooth and strong. Could the impeller be slipping on the pump shaft??? Is there a bad type of DE or some chemical that clogs the filter panels???
Yes, it's possible that the impeller could be stripped if it is the type that threads onto the shaft. You can tell by whether or not it has a screw in the head of it. If it does, the screw is what holds it in place. If there is no screw, it is threaded and you need to be sure it is tight.
The problems that could come from clogging the filter wouldn't be related to a type of DE but more with an excess amount of it being used. You can close off filter area by using too much DE. This is a condition known as 'bridging' in where the area between grids is full of DE. Thus allowing the grids, in effect, to touch. The DE is designed to create only a thin coating over each grid. I often have opened a DE filter to find that there is bridging and this reduces filter area and restricts flow. Mostly I see this where people backwash their filter and then add too much DE to replace the amount that was washed away.
The only other chemicals which can clog a filter are phosphate removers, or other sequesters, which work by collecting whatever they're designed to collect (metals, phosphates, etc.) and they can cause the filter to become clogged. You would know if you had used one of these. Backwashing or cleaning the filter will fix this.