I can't manage to change that attack of a trombone sound

I have a soundfont with a really nice trombone sound. The only issue is that it's a noticeable "wa-wa" sound, i.e. it fades in. Basically like a fast crescendo. This makes it nearly imossible to write fast passages, because you can't hear the notes. I went to change the attack on the synthesizer using the SC4 effect but a.) it was already at its lowest setting, which is stupid because there was a huge "wah." B.) changing it didn't affect playback. How do I fix this?