One more reason could be non standard gp which might fail to match the master file and in turn the WL. Sometimes even diff brands can give us this problem! I had trouble using waldent cones compared to diadent which is slightly better!
It depends. If it is wide canal then you may fit 6% directly. But if its narrow canal and u have done 4% enlargement then 6% cone wont go. Ideally one should enlarge the file to the same size of gp
