On the core server launch LANDesk Configure Services or with the 32bit console >Configure Menu > Services...
Check the Days to keep inventory under the Inventory Tab. Perhaps this was set to 1 day. Default I believe is 30 days.
Have you looked at the Duplicate Devices and Device ID on the Inventory tab? Click Configure > Services > Inventory
There are a couple of possibilities, one being that the maintenance routine is being affected by duplicate MAC addresses or that you have duplicate device IDs as already mentioned.
Take a look at these documents and see which fits the best. Both are very detailed.