That's correct, most likely, no information for the computers was found from Active Directory. As you are using an inner join to link the database table tblADComputers, the asset also needs to have a record within that database table. Therefore, I would recommend to use a left join to join the tables as seen highlighted below. This will show all your assets based on the criteria and will also show the AD Description, if found.
Select Top 1000000 tsysOS.Image As icon,
tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tblAssets.Username,
tblAssets.IPAddress,
tblAssets.Firstseen,
tblAssets.Lastseen,
tblAssets.Lasttried,
tsysOS.OSname,
tblOperatingsystem.Caption As FullOSname,
tblOperatingsystem.Version,
tblOperatingsystem.InstallDate,
tsysIPLocations.IPLocation,
tblADComputers.Description
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tblOperatingsystem
On tblAssets.AssetID = tblOperatingsystem.AssetID
Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
Inner Join tsysIPLocations On tsysIPLocations.LocationID =
tblAssets.LocationID
Left Join tblADComputers On tblAssets.AssetID = tblADComputers.AssetID
Where tsysOS.OSname = 'win 10'
Order By tblAssets.Domain,
tblAssets.AssetName