Are you seeing this for a large percentage of your library, or for specific songs? If it's specific cases, I don't know how iTunes does the matching for album art, but it is not matching for the affected songs. And it's likely to be more involved than simply matching by song, artist, and album names.
What I have done for missing artwork is find an image of the album elsewhere. For example, look up the same album in Amazon's MP3 Store. In the listing for that same album, click/hold and drag the album art image from the browser window to the Mac's desktop. The image file downloads and appears on desktop. Then, open iTunes window to your Music library and do a Get Info on song with missing (or wrong) artwork. In the Info window, go to Artwork tab. Drag the album art image file from desktop to the song's Info window in iTunes, and drop it there. The album art appears there. You can select all songs from an album and do a Get Info (on the entire selection), if you need to add the same album art image to multiple songs at the same time.
You can also do a screenshot to get the album art image, if image file cannot be dragged to desktop. Just show the album art image on your screen anywhere, in any app. From the keyboard, do a Cmd-Shift-4. The mouse cursor turns into tiny "crosshairs." Click/hold and drag any portion of the screen (such as precisely around the album art), to capture that portion as a screenshot. The screenshot image file appears on Desktop. Drag it to the song's Info window in iTunes.