Wracking the brain as it's been years since doing a chicken.
There are sort of two lengths of feathers, all you are doing is trimming the longer flight ones to a similar length to the rest, not cutting them right off.
If in doubt cut less off, you can always trim more if they are still...
Mine couldn't work it out for a while either. I actually just put a block under to make the slope less and that solved the problem. I removed the block when they were a bit older.