Not knowing the welders you're using: make sure the polarity is set to DCEN using flux core wire (for those machines that can be switched), try the wire feed at only 1/4 of full speed with the heat set on high (I have a dc and ac 110v machine that won't weld faster than 30% wire feed using flux core), try to use multiple pass wire like Hobart E71T-11 (most GS wire won't weld to a previous weld).
Listen to the sound of the arc, should sound like frying bacon. Clean metal is not a must, flux core cleans as it welds, but cleaner does help. Some welders are setup to weld aluminum so their wire feed is fast at the higher setting. Flux core needs slow and steady to penetrate. I started with flux core machines and was surprised at how fast a good dc welder using gas will weld. I guess that's the difference between welding with solid wire and tube wire. With the limited power of some 110v welders, I've gotten solid flux core welds at very slow wire feed, usually 1.5 to 2.5 (on a 1 to 10 setting machine) depending on material thickness. Travel speed needs to be correct also to get a good weld. Too fast and it won't penetrate and sit on top, where too slow will burn through the base material. You want just fast enough that you don't burn through. Good luck.