Recovered files on first try, subsequent tries failed

I used Recuva to retrieve data from my corrupted hard drive. The hard drive would not run from a boot disk, so I slaved this drive into another computer, both running XP. I was able to scan the drive in Recuva which found 40% of my files. I recovered one file to the working hard drive and it worked. Then, I recovered one sub folder, with 50% success. After this, I was not able to recover anything else, even similar sub folders containing the same file types.

I scanned using the setting to look for non-deleted files.

I have restarted the computer and reinstalled Recuva. Every time I scan on the drive in Recuva, it finds fewer files and nothing is recovered. Is it a limitation of the freeware version? Could this be due to a degrading hard drive? Or something else?

Thanks.