Magical boots are sometimes not free, and sometimes can't be bought back after time completion

I've had this incident happen on numerous occasions, but I haven't figured out why yet. Lately, I've been playing a great deal of Klepto on Kayle, and so my item slots are often filled with random consumables at around the Magic Boot completion time.
Usually, when I sell off or use these items to free a slot, I get my magic boot. But sometimes, I don't get any free boots. Then I find myself having to go back to the shop to buy it.
Recently, I've had an even worse experience happen. Not only did I not get my boots, but I couldn't buy them in store. So I could only get any type of boots by saving up and directly buying the Tier 2. I don't know what caused this to happen yet. I intend to do some of my own testings in Practice Tool, but I don't know when I'll get around to it. I wanted to at least document this issue incase anyone else experienced it or had an answer. Thanks in advance for any help anyone is able to give.

I wanted to provide a picture but it seems I'm unable to do that on this post.