if you successfully added some artwork following the instructions, then there should be no problem adding more.
Just make sure that all the artwork you are trying to add is in a format that iTunes recognizes.
Take a look at the successful additions and figure out if their artwork is the same format as the failed ones
Followed the instructions in iTunes Help - some albums with no artwork I was able to add but others will not work. Some are very old albums (originally LP vinyl) but some are not so old. Is there a trick to adding the artwork?