Been fighting with my Millermatic 185....
Make sure polarity is correct. If I recall, flux core is often operated at opposite polarity from gas shield wire
Make sure the power connection nuts are tight
As above, make sure the gun / cable is "shook out" and unkinked. Often I have a "glitch" I just shake out the cable and "turn" it 1/2 turn or whatever and it feeds good
Make sure you have the right type feed rollers. I think I accidently got flux core rollers on mine once
I use whatever the tip dip stuff is and it seems to help
On a side note, for the type welding I mostly do, straight CO2 serves me well and it's incredibly cheap.