Album artwork not always appearing on IPad

Quite a few of my albums have been ripped from purchased CDs. These don't always find artwork from the database. So I scan the album artwork and place that in a folder in My Music (so I know where it is). Then I put the ripped music tracks in MP4 format in folder called the same as the Album name. Usually this album is put under a folder called after the Artist. Then I use the Add Folder to add these folders. They then appear in my Music in iTunes. But the Album has not artwork at this stage. I then RH click on the album in Music and go to Info and bring up the info window that has a window for the album artwork. (Blank at this time). I then drag the album artwork from my PC folder onto the blank artwork on info window. Then when I synchronise the Album to my IPad I see the artwork on the IPad. Well at least this has worked nearly every time. But I have just ripped a Mozart CD and when I put the album on the IPad no Album artwork is shown? I think one of the areas you have to watch is that all the Meta tags on the tracks etc... match up. But I may be wrong? Any ideas why the method I have been using successfully, doesn't work on this Mozart album of symphonies. I have about 5 of these CDs and was going to RIP them and add them manually. But I am confused why in this case the album artwork doesn't appear on the IPad like most of the other CDs I have Ripped and manually added along with their artwork. Any ideas?