It can be a pain and a long wait if you have a lot of covers.
1. First delete the cache folder in /wiiflow directory
2. Second check for any covers corruption in /wiiflow/boxcovers
3. Third launch wiiflow and rebuild the cache
4. If you get same problem try another wiiflow dol (like official 4.1 or 4.2 or 1076mod 1054mod...)
when all you covers get caches you can replace the dol by the one in the masterpiece pack
p.s. As mentioned before give it some time, it may freeze too
good luck