I found that FIT files that you load onto the Garmin Edge 20/25 - in my case a 20 - through the newfiles folder need to have time properties and two courses on the device cannot have the same start time. Downloaded or hand-drawn tracks often lack time properties and converted tracks may have identical, default time stamps. Make sure that the files contain correct time tags and FIT files load onto the device nicely by adding them to the newfiles folder and switching the device off and on.
First make sure that your GPX track has correct time stamps, for example with GOTOES 'Add Time Stamps to GPX Files'. Make sure to set the Activity Start Time.
After that, GPSies Convert can convert GPX tracks to proper FIT files for the Edge 20/25 as long as you set the Speed (mph) parameter to 0 (zero). Use any other value or leave the speed parameter empty and GPSies will use a default start time (2010-01-01T00:00:00Z) that causes upload of a second file converted with GPSies.com to the Edge 20/25 to fail.