I had to build my own lock (install tumblers, etc...) with the Sport Top Rack.
A possible issue would be the mounting of the tumbler barrel in to the case. When your key is inserted, is it lining up perfectly with the lettering on the case? If not, you may want to loosen the screw on the inside and twist it a little for alignment, if it is off, the catches in pic 2 here could be hanging up the springs.
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Another potential could be the tumblers are off by one count and it is catching when the button is pushed. Just a guess, but something to look at. You can pull the lock barrel out (carefully with your fingers covering the tumblers and springs as you pull it out) and run you finger around it with the key inserted, if you feel any one of the tumblers protruding or sunk in (we are talking a couple millimeters between tumblers) you may need the next size up or down. Mine was very finicky, it took me 3 tries overall to make it work smooth.
If you are not comfortable with messing with the tumblers, any dealer should be able to look at it under HD warranty as well as long as the lock was installed when you received it.