Depends on the map, if you saved them on slate, then try and load on slope, they won't load, or appear on the save list. If this isn't the issue, i think you are having something that maybe hiding files from your blockland, which is a major virus, or a bugged blockland.exe.
1. Scan for any viruses, or Malaware
2. Make sure your save files aren't corrupt
3. Re-install your blockland (Last resort)