Can't add new artist to custom iTunes radio station

Hi,

I've been trying to solve this for one year now.

When I create a new artist station, say a Metallica radio station, the I click on Add artist, song, or genre then I select say Lady Gaga and try to add her to my radio station, nothing happens. No new artist is being added to the station. This is really annoying.

Sometimes, it works, but even when it does, it doesn't. I mean it sometimes lists the artists I added (like 2 out of 100 tries), but even then, it only plays music similar to the main artist of the station. In the above case, for example, if I let the station play 500 songs (yes, I did this many times, checked on last.fm), I'd only get music similar to Metallica (thrash metal, heavy metal, hard rock, etc), and absolutely no music similar to Lady Gaga, although it tells me the station is based on both of them.

Same thing happens for genres and songs.

As of this post, I'm using iTunes 10.0.1.26 on OS X Yosemite 10.10.1 on Macbook Pro Retina 15-inch Late 2013.