Created attachment 206327 [details]
SVG file with two paths with implicit and explicit lineto commands

Some shapes I created with Inkscape are imported, some not. 
The solution is in the path data (XML tag <path>). 
Inkscape depicts the path as a sequence of lines, which starts with an 'M'.  Each line is depicted by a pair of coordinates of its end point. The SVG standard treats them as implicit lineto commands (see http://www.w3.org/TR/SVG/paths.html#PathData, chapter 8.3.2). But GIMP expects explicit lineto commands (L or l) for each single line. If they are missing, only an empty path is imported.
To show this effect, I attached the file star.svg. Path1 is a proper path with implicit lineto commands, but not properly imported. Path2 gets properly imported with its explicit lineto commands.
Because implicit lineto commands are W3C standard, GIMP should be able to import such paths properly.

reproduction:
in Inkscape (0.48.1): 
1. create vector shape with 'Stars and polygons' tool
2. in Inkscape: save shape as plain svg (to be standard compliant)

in GIMP: 
3. open paths dialog
4. open path menu or right-click, choose 'Import path...' and then the file to import 
5. path is not imported. In case of the attached file star.svg, only path2 is imported.