This kind of issue is mostly the result of Lansweeper not getting the serial number from the monitor (or more correct, from WMI not getting the serial number from the monitor), and since the serial is the main monitor identifier, Lansweeper does not regonize those monitors as an asset.
But they do appear on Display->Monitor, they just are not assets, they do not appear on the summary page, etc.
So my first advice is that you donwload an hardware info program, like hwinfo, and check if both monitors report the correct serial number, we have a lot of older monitors that have that issue, and your Dell 1707FP is not exactly new...