A couple of thing I can think of is:
- the zip disk you put in is getting pretty old and is damaged (goes very quickly with zip disks)
- the zip disks has the wrong formatstructure
- you use 250 MB zip disks in a 100 MB zip drive
To get a zip disk out, you can use a paperclip and fold it open and stick it in a little hole, push hard and it should come rigth out.