Re: Dull& Worn floors after 5 years !
I would recommend sanding the finish off if the Minwax you used was a water based poly, or a light sanding if the Minwax you used was oil based. I recommend OldMasters brand oil based poly. It has excellent self leveling characteristics and dries to a very hard finish. I recommend 3 coats. Our kitchen floor was refinished about 15 years ago and still looks great.
If you wish to use a water based finish OldMasters also makes a water based poly but I have never used it. With a water based finish you should be able to just lightly sand the floor and apply new coats.
Be sure you live your life, because you are a long time dead.-Scottish Proverb