The o/h door in my pole shed has not closed tight to the floor since the floor was poured - there was always a slight gap. Wasn't a big issue to me when I was only using the shed as a storage building, but now that I am in the process of insulating it as a shop, the door has to close better.
Some crawling around on the floor w/ a level and feeling the contours of the door opening w/ my hands turned up a small bump in the concrete, next to one of the tracks - maybe a 1/8 - 5/32 inch high.
I did some poking around in the tool sections of my local hardware emporiums, and found a attachment for my angle grinder - a Diamond Cup Wheel by DeWalt (DW4774).
Sprayed some water on the spot (to minimize the dust kicked up), and hit it w/ the grinder. There was a small area where I couldn't get in to w/ the grinder head - I took that out by chipping away w/ a hammer and cold chisel. The hump is now gone, and the door now sits tight to the floor.