Disaprearing implements?

I found myself buying new implements, and always thought that I probably ''only had 7 and not 8'', etc. But this morning, I only have 4 air boom spreader, yet I know yesterday I was fertilizing with my 8 tractor at a time. Is it just a bug that I have, or do they get to disapear out of use at some point ? I don't get it at all!