I still think you don't test properly. But assuming you're doing right, the only other possible I can think is the function which is called when an explosion happens, is not triggered.
Compile this plugin, I've added debug text.
1) Make sure you're using this plugin (amxx list -> you should see version 2.1.0-debug)
2) In game just buy an HE and throw it on the ground and see what happens.
If nothing happens, check the AMXX log (amxmodx/logs/) ; in latest you should see output (at least first line, example below).
E.g.
Code:
L 05/14/2014 - 17:47:23: [Untitled.amxx] Orpheu Hook: 1
L 05/14/2014 - 17:47:43: [Untitled.amxx] CGrenade_Detonate3 : 317
L 05/14/2014 - 17:47:43: [Untitled.amxx] m_usEvent = 25
L 05/14/2014 - 17:47:43: [Untitled.amxx] find_knockback_entities = j = 0, ent = 317, owner = 8, origin = 1608.2 2595.0 -7.9 radius = 300.0, damage = 75.0, ff = 0, mode = 1, wallhit = 0
L 05/14/2014 - 17:47:43: [Untitled.amxx] find_knockback_entities = j = 1, ent = 317, owner = 8, origin = 1608.2 2595.0 -57.9 radius = 300.0, damage = 75.0, ff = 0, mode = 1, wallhit = 0
L 05/14/2014 - 17:47:43: [Untitled.amxx] find_knockback_entities = j = 2, ent = 317, owner = 8, origin = 1608.2 2595.0 -82.9 radius = 300.0, damage = 75.0, ff = 0, mode = 1, wallhit = 0
L 05/14/2014 - 17:47:43: [Untitled.amxx] find_knockback_entities = j = 3, ent = 317, owner = 8, origin = 1608.2 2595.0 -82.9 radius = 300.0, damage = 75.0, ff = 0, mode = 1, wallhit = 0