Sometimes can't drag album art OUT of iTunes

Hello. I am one of those people that is very meticulous about correct album art showing up for all my songs. As such, I sometimes scan the original artwork to import into iTunes. THAT works without fail. I never have a problem getting artwork into iTunes...But there are situations when I want to have that artwork back to work with it (slight cropping, color correction, etc). Sometimes I can do this just fine...open the Info window, grab the artwork, drag it to the Desktop, done. But sometimes this fail to work. I thought it might be because I am trying to drag out artwork that was downloaded from the iTunes store rather than artwork I pasted in myself, but I've found that it doesn't matter. Once it stops working, it doesn't work for any artwork, no matter if I made it or not. Eventually (and I don't know what the magic is that makes this happen), it will start working again, letting me drag and drop as I please. What gives? Anyone else find this? All I can find on the Web is information about adding artwork to iTunes, not pulling it out. Please help.

It's been happening across multiple iTunes release versions. My specs...

• iMac running 10.10.5.

• 2.9 GHz Intel Core i5

• 8 GB 1600 MHz DDR3

• iTunes 12.3.2.35