Inside the plastic attachments, there is a guide. If you don't pay attention to attach it 100% aligned and it goes in crooked, it will get stuck. Then, to remove it, you will need a tool and because it is particularly fragile, this guide will break, rendering it useless afterwards as the collar no longer attaches. Unfortunately, it is very easy to attach it incorrectly. I had the same issue with another Small one I had. The Medium one is much more durable and in the 2 years I have had it on my dog, it hasn't suffered any damage.