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.