Okay so for anyone who actually comes across this problem, I done some fiddling, it seems that as long as I can put it in at the right bent angle it goes in, but otherwise it must hit something inside, I hear a very distinct "tink" noise that the dark slide is hitting, The Darkside is one of those plastic like dark slides, not the metal ones, not sure what the differences, I have two different backs a 6 x 7 and then the 6 x 12 now, and the designs insider different, the 6 x 7 has a backing plate that is connected to the actual device, the 6 x 12 has a backing plate that actually lifts out of the device, I also noticed that the 6 x 12 has an additional metal silver roller, the first time I put the film over the roller, this time I put it under the roller, and this seems to have made it a little easier to get in however the issue still exists slightly, and I just have to be careful about how I push the dark slide in, the other factors not do this at all no matter which Darkside I use etc.

At least it's usable and I don't have to return it, just wish it function properly from the beginning.