In Lord of the Flies, why can't Jack get the pig to stay on the spear?

As Jack experiments with hunting the pigs, he
troubleshoots his difficulties along the way to discover the best possible way for
catching and killing a pig.  One of his issues is that he cannot lodge the spear into
the pig; the spear keeps sliding out. 

When Jack meets up
with Ralph in chapter three, Ralph is incensed that nobody is helping him build the
shelters, fully insinuating that if Jack cannot kill a pig, then the least he could do
is to help out around camp.  Instead of acknowledging Ralph's frustration, Jack makes
excuses for his inability to kill a pig:

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"But I shall [kill a pig]! Next time! I've got to
get a barb on this spear! We wounded a pig and the spear fell out.  If we could only
make barbs--" (51).

Jack's
quest to improve his hunting technology reveals his ability to think critically and
improvise.  He will devise a barbed spear, and the next hunt will prove
successful.