At first it seemed like angrily shaking the yukkuri wouldn't loosen the shell from its head. Then on page 15, the yukkuri shrugged its own shell off for a moment because it had shifted and was covering its face; but, by the very next page, 16, nay, the next panel, the shell, which was previously rather difficult to move for the koyukkuri, is miraculously back on its crown.
At this point, I think the shell is akin to glasses and this yukkuri is absentminded about its crutch. It knows enough to take it off and fix it, but doesn't notice that its shell - its accessory, a literal personal extension of the yukkuri - is the cause of its inability to move quickly or jump high. It fixed it by taking it of and putting it back on but doesn't seem to even realize what it did.