So you got yourself a brand new, shiny reel and loaded it up with some of your favorite braided line...only to find out the drag is slipping and you can't land any fish! Well, 9 times out of 10, we see that the braided line is actually sliding on the spool arbor...not the drag failing.
This can be checked by putting a felt mark on the edge of the spool above the line, and pull the line out. You should be able to see the line moving but not the felt mark...a clear sign your line is slipping! Sometimes the line was put on tight and only slips lightly until you get in the cold. This can be checked by putting the reel in your fridge freezer for ten minutes or so. This will shrink the spool ever so slightly and allow the line to spin on the spool.
All this being said, to eliminate this issue, you must either put a couple layers of monofilament on the spool BEFORE putting on the braid, or add some duct tape to the spool, again before installing the braided line. This will keep it from slipping on the spool arbor.