In JJS Skill Builder, the usual way to make a grab hitbox work is to add the hitbox first, then connect it to the grab so the target gets “stuck” when the hitbox lands. A recent tutorial shows the setup as: spawn a character block, open Configure → Skill → Edit , add a hitbox , set a weight to 0.01, add a grab , choose right arm , scroll to last hit and set it to 0.1, then add another weight and leave it at 1.

Basic setup

The same tutorial also notes that if you want the grab to trigger as part of a full move, you can add a skill after the grab is built. It describes the grab result as making the opponent follow you once the hitbox connects, rather than just dealing normal damage.

If it is not working

A common issue is that the hitbox is dealing damage but not properly attaching the enemy during the grab. Another recent discussion asks exactly about hitboxes failing during grabs, which suggests the main fix is usually the grab connection and hitbox timing, not just adding raw damage.

Simple version

  1. Open your skill in Skill Builder.
  2. Add a hitbox.
  3. Add a grab.
  4. Tweak the grab timing and hitbox weight.
  5. Test until the enemy stays attached instead of being knocked away.

Example

If your grab lands but the enemy slips out, lower the hitbox weight and make sure the grab is set up on the correct body part and hit timing. That is the same idea shown in the tutorial that uses 0.01 for the hitbox weight and a timed grab connection.

Extra note

A separate guide on JJS hitboxes and connect blocks also points to hitbox/connect/velocity blocks as part of the same skill-building system, so the grab setup is usually about combining those blocks correctly rather than using hitbox alone.

TL;DR: add the hitbox, then the grab, then tune the weight/timing so the target stays attached when the hit lands.