Thanks a lot, everyone. Much appreciated. I think I know what I was doing wrong and it has to do with aligning the film properly at the start.
There is a short video in utube on loading 120 in a SS reel that also helped me "get the hang of it".

I can now load a "sacrifice" cheap film without problems, both in and out of the bag.
Got a few test rolls to go through this weekend, will try it again.