I have a master 140, and it also exhibits this pin "locking" behaviour, sometimes its even the last pin with my pick behind it

I just let the pins drop and start over... So I cant tell you what it is...
I believe (and also someone has dissasembled the 140 on this forum and posted piccies somewhere) that the 140 has 3 spools and 1 regular pin...
Shouldnt be too much trouble with practice tho.... try picking in the order of (starting from back to front 4-1) 3421, thats how I open mine...
GL..