When you report a bug like this, it's helpful to explain how exactly it 'doesn't work'. Does it not increase the damage if it succeeds? Does it not damage the caster when it fails? Does the ability option just not show up at all?

Damage done depends on the defence of the enemy.

So does it happen when you strike the same opponent ? (or an opponent with the same defence ?)

That can happen is the gambler's strike rolled a 18 (that transform in a 36 - a roll of 11 in defence = 25) and you rolled a 26 for the crushing blow (26 * 1.5 = 39 then a roll of 7 in defence)

That would be odd if that thing happened several times.

If you have 10 in attack you roll a die to know if you'll get 10 in attack. In WoM the dice ranged from 0 to 10, in FE we can't be sure but it seems the range is more compact around the value (you'll rarely roll a 0 if you have 10 in defence. In WoM you have 1/10)

The same thing is for defence.

Same here. Something's fishy with gambler strike.