There are several reasons to that, so first of all let's list what SparkShare will not propose for upload:
- Levels that have been downloaded with SparkShare
- Levels you did not save and that are only autosaved
- Levels that are not named (meaning, not saved)
- Obviously levels that are not on your computer but only on the cloud (just save them locally)
These 4 cases are intentionally dropped from the share list. The first point is open for discussion if the community feels like it
Now there is another kind of levels that do not appear and that is more of a bug. Some older levels aren't in the same format like the newer ones made lately (2015? Only TD could answer)
Usually these levels pass the 4 above mentionned points but still aren't proposed for upload. This is due to the fact that SparkShare need to extract the world name from the binary file by reading it but fails as the format doesn't allow SparkShare to read it.
There is a solution though, that has proven working until now: Open up Project Spark, edit that level. Once in the editor, just do a slight change, nothing great, just change something like create a tiny elevation somewhere, rotate a prop by 1 degree, whatever. Hit the menu and save your world. Once done, relaunch SparkShare and the level should be available.
If not, then we've hit a yet unknown problem and I would be pleased to help you solve it. Take contact through pm with me so that we can make this work