Re: Line Wt.
The line may not be the only problem.
The reel can also be a problem. It may be too small to hold a 6 wt line with 50 yards of 20 lb backing.
The fly shop created a mess for you. If they try to load a 6 wt line on, watch as they do it. Make sure they do not cut off the back end of the fly line to create extra space for the 6 wt line.
Also make sure that after the line is loaded, you have about 1/4 inch of "head space" or clearance between the line and the top edge of the spool. That is prevent binding of the line during fishing.
You are never as careful winding line in during fishing as they are in the shop when winding line. You will need that clearance space while fishing and to remove the spool.
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy