I have also experienced this, where disks with enough read/writes don't save the data. In my case my floppy actually saved one of them out of the three I did, and weirdly not the latest one either. I moved it onto the PC, onto a ZIP and once I'd gone back and collected the other two, it was completely gone from the ZIP as well. Out of curiosity I put the two data blocks I'd just collected into the ZIP as well, then ejected the ZIP before putting it back in the PC and now all three of the data blocks had completely vanished.
So it seems that data can disappear from both floppies and ZIPs if you eject and put them into slots between putting data on?