Album Art not synching, I have 1 iphone 4s, 1 ipad G4, 2 ipod touch G4 and 1 ipod touch G3

Hi, I have all the aforementioned devices listed, all with the latest ios version availabe for the device and allways have the latest version of itunes installed.

when sysnching if the album art is provided by apple, then it will synch, but if is manually added it, some time it will be transferred and some time not, do not have idea of what can be the problem. Please help. BTW and the album art that is/not transferred is differen in each device some time some song will get it on the ipod, some times it will not to go to the iphone and so on.

Pleae help