Question on Thrawn's basic

Duramater

Several times now (maybe 30% of the time), I use Thrawn's basic, either directly or through a counterattack, and it does not apply speed up. I thought this was irresistible and independent of tenacity (since its a buff, not a debuff). Observed regardless of the opponents' buffs or debuffs.
Am I missing some other mechanic here?

C1pher_Five

It should work in all but two cases: 1) it doesn't hit or 2) target has buff immunity.