Perseverance (Vanguard Belt) can't get Onslaught when I get fortity

Rarity: Unique
Perseverance
Vanguard Belt
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Requirements:
Level: 78
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Item Level: 73
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+282 to Armour and Evasion Rating (implicit)
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8% increased maximum Life
+37% to Cold Resistance
1% increased Attack Damage per 200 of the lowest of Armour and Evasion Rating
17% chance to gain Fortify when you Stun an Enemy with Melee Damage
You have Onslaught while you have Fortify

pls help me!
why i can't get onslaught when i already get fortify?

I bet you get Onslaught when you have Fortify. What you dont get is the icon, because Onslaught is granted by an item and not by any buff.

As Lud points out, you do have the buff, it doesn't show up as it is an item granted buff not a support gem granted buff.