We replaced the stock hotend in our Orion Delta printer with the E3D v6 hotend. Our first test print went great; however, we quickly discovered that the new hotend only seems to work with some of our filament. The filament should all be basically the same chemically - all of it is 1.75 mm PLA, and it all worked with the old hotend - but certain filament now simply won't go through the extruder - it just gets stopped there, like the extruder wasn't melting it to be able to push it through the smaller opening on the end. One of the colors that works is from the same manufacturer as one of the ones that doesn't, to show how weird the problem is.

Any ideas why this might be? How could we fix this?