You really need to tell us a little more.
Gas Shielded FCAW ? What wire Diameter ?
Self Shielded FCAW? What wire diameter ?
Are you doing code work ? What is the code ?
What type power supply..? 110-v or regular CV 200+ Amp ?
I weld 16ga Self-shielded FCAW all the time with my little garage Miller 140... It works great for little jobs like BBQ's and stuff... It will go anyplace a 110 plug will reach......
There definitely are some tips and tricks for welding thin material with FCAW.... Things to do that will reduce penetration, which it appears you are having a problem with.... But they type of power supply and Gas V Self shielding have different advice for doing what you want..