I picked up some aftermarket reproductions at a gunshow awhile back..... and had the typical issues of loading. After watching a few vids and reading about how some guys try and rectify the problem, I tried a way that really helps alot, matter of fact, it will get as good as it will get.
I loaded up some dummy loads, and worked the loaded clip in my bench vice...
Standing the clip with cartridges up, I worked one end of clip to the other gently closing the vice jaws on the clip lips.
Next layed the clip with cartridges flat, I worked the clip in the vice jaws gently pressing the lip of the clip from one end to the other.
Basically what I accomplished is the channel where the cases slide into.. having a slightly looser fit, causing the cases to bind.... The channel now is tighter and now the cases do not bind and slide down easier in the clip.
It works very well...