The answer depends on how the asset was opened for edit.
If you opened an asset for edit and modified the asset then you have probably forgotten to commit the asset when finished (rightclick asset/edit/commit).
If you opened the asset for edit just to look at it but made no changes then you need to revert the asset (rightclick asset/edit/revert or revert to original).
If while importing a cdp file you allowed the new asset to overwrite an existing asset this will leave the asset open for edit and it will need commiting or reverting depending on whether or not you really wanted the asset overwritten. (sometimes a newer version from third party sites can be updated but the kuid is not updated to a kuid2 version).
Also there is an option in content manager (CM) go to file/settings/miscelaneous and there is a check box for "automatically commit when installing CDP's" make sure this is checked.
Thanks gremlin, forgot that one. If this is the source of the assets left open for edit then they will need to be committed, if you revert them they will disappear and you will have to redownload or re-import them.
As I said above it depends on whether or not you intended to overwrite the asset. Commiting an asset usually results in the asset being marked as locally modified which is a big no no for multiplayer.
Well i have to Commit things every time i;m in CM i think it's do to me using my raildriver.
So now i check open for edit before i do anything with Trainz.
The quickest way to commit open for edit is to right click on the top one go down to slecet all then hit CTRL + M at the same time.