The gear setup in the first stage feeder is setup such that sometimes the roller is connected to the motor and therefore won’t roll freely and appears to be jammed,
This usually works fine but I find that occasionally when filament is low on a spool the gears to not disengage properly and so the roller won’t spin freely and so the spool climbs up and hits the top.
If your problematic slots have low filament on the spools then my 500g solution mentioned in this post, or possibly other weight or ‘AMS Saver’ solutions mentioned might help.