A lot of the levels in Guardian Tales feature hidden mechanics. We believe that giving out spoilers and walk-throughs ruins part of the fun, but we also understand it is very frustrating if you are unable to fully clear a level, so here are some tips:
Hookshot is an ability that can only be used in certain spots.
Please also know that in order to use Hookshot you must be at the same height level as the target.
In the case of World 3-6, climb the stairs in front of the treasure chest and face right to see this option.