Some reasons why projects fall between the cracks:
Terminology. People are odd about this. They think that the Marketing Department is supposed to handle, well, marketing and that the IT people have an internal monopoly on IT stuff. When there are exceptions that are not broadly communicated, then things get lost, even by members of those departments.
Blurred Lines of Responsibility. This is the easy one. Blurs create conflict and miscommunication.
Poor or No Follow-Up. If we all discussed it and reached agreement, that means it is magically done. Right?
Sabotage. Someone wanted the project to fall between the cracks. It is tempting to believe that this is the reason. Tempting, but seldom accurate.
Turn-over. Ed forgot to tell his successor about the project while he was packing his bags for retirement in Tahiti.
Differing Priorities. The project only seems to have fallen between the cracks. It was placed there for safe-keeping and will be retrieved after other priorities have been addressed.
Loose Language. When you said that we would handle it, I thought you were using the royal we.