Magwell tightness is set by a few things: relation of front trunnion to opening of magwell(forward or back set during riveting and can usually also be judged by top cover fit), selector stop plate(sometimes too thick or thin), mag catch, rear magwell cut/selector stop relation.
Since you said you can rock the mag backwards causing it to sit higher it could be a couple of things... the mag catch, the carrier rails, or the FCG pushing the carrier up on sloppy rails.
Remove the FCG, reassemble, try to feed it. Also, take a pic looking straight down into the magwell and another as close to parallel with the bottom of the receiver looking straight back towards the mag catch. If it's a Century build use your warranty. I'd help you in person but I don't have a TIG and if it's the mag catch that's like asking me to pull a tooth out. Measure the gap between the magwell and mag catch with calipers if you have them.
Since you said you can rock the mag backwards causing it to sit higher it could be a couple of things... the mag catch, the carrier rails, or the FCG pushing the carrier up on sloppy rails.
Remove the FCG, reassemble, try to feed it. Also, take a pic looking straight down into the magwell and another as close to parallel with the bottom of the receiver looking straight back towards the mag catch. If it's a Century build use your warranty. I'd help you in person but I don't have a TIG and if it's the mag catch that's like asking me to pull a tooth out. Measure the gap between the magwell and mag catch with calipers if you have them.