Its been at least a year since I went looking for a "data recovery" program when a drive failed.
Incidental information #1 is that all the useful data recovery programs for hard drives were too expensive to be worthwhile (for the data I wanted to recover). There were a couple of data recovery programs that were less expensive that might have been affordable enough to try using on memory cards up to about 1GB, but the "small programs" wouldn't handle hard drive sizes.
Incidental information #2 was the advice that:
Memory cards used in cameras may lose format, and this is a "usual cause" of individual images - or entire cards - that aren't readable.
It was recommended that any "memory" used in a camera should ONLY BE FORMATTED BY THE CAMERA in which it's used.
The caveats are that it was quite a while ago when I ran into this info, and the info may have been "well aged" by the time I found it.
Both cameras and memory cards have changed some (in some cases "improved") since then, so a reformat of a card in a reader, using the computer format, may have a better chance of working. Some of the "cheaper" cameras now available may have decided that the format utility isn't needed(?).
Your Nikon almost certainly can do a memory card format of an inserted card, and that would still be the recommended method in your case. The memory is small enough that you can easily dump the information that's on it, and a format in the camera is the most reliable way to assure that the memory is "clean."
If you're losing individual images, removing all the readable pics and running a format on the card - in the camera and using the camera format utility - may avoid having increasing numbers of lost pics in the future.
If you have "unreadables" still on the card that might be worth the effort, looking for a data recovery program might find something useful, and reasonably affordable if the need justifies any expense at all, especially for a small card like the one mentioned. (I probably wouldn't bother, unless I knew there was something really precious on the card.)