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Most likely, there is a loose connection at the feed end or coupling for the section of track you are having trouble with.
Loose connections are often exposed when more load is added to a circuit, such as an additional light bulb.
To repair this, make sure the wall switch is in the OFF position. Open the small cover at the feed end of the track. Make sure the screws where the wires connect to the track are tight. If you can access the ceiling box, make sure any splices in that box are tight as well. If the track has any couplings in it, meaning it's two or more sections joined, remove the covers from the couplings and check those screws for tightness as well. You can also check for loose connections in the wall switch box.