iTunes dropping album artwork

Does anybody know why iTunes continues to drop album artwork? the artwork shows up in the "Now Playing" Box but not in the main menu of albums. It is a constant pain to keep re-stocking album covers.

Any advice is appreciated.
Zd542 I responded to your suggestion with a "Thank you I will try that" but I did not see my post in the thread - so, Thank you, I will try that! (again).
Puerto, You can open properties by right clicking and selecting "Get Info". You can do that to one song or more if you mark them. Sometimes only one song has artwork. I don't know why, perhaps this song was highlighted/selected when I was dragging artwork into album (blank) icon. Strangely, when I click on "Get info" for the album artwork/icon it has none.

Another confusing thing in Itunes is fact that I cannot see my albums as tiles. Remedy for that is to set "Smart PlayList" with "Genre contains xxxxx" where I set xxxxx to genre like "Classical" etc. Later I added "- " in front of the name to have them appearing together (not mixed with other playlists). Playlist still allows to change properties but does not allow to delete - good protection.
Thanks Kijanki - Like you, I have some albums where only track no. 1 has the artwork so now I know how to remedy that.

All of my artwork is showing now since spending hours re-populating the artwork from the internet. It looks like all the ones that I just re-populated have artwork for each track. That is a new development so things are improving.
That's OK. The delay between when you post and when your post shows up can make things very confusing. I was in a bit of a hurry when I typed my response. The reason I suggested you try all of that, is sometimes things like external HD's don't fully boot up fast enough. So if your OS and iTunes get ahead of your HD it can cause all kinds of problems. Missing artwork is a very common result when this happens.
Some of the other posts talk about tagging issues. They're absolutely correct. That can be your problem, as well.