@turquoise-- What dose are you on?
Amitriptyline wasn't helping my IBS at lower doses either. But my doctor had me slowly increase my dose to 100mg and then I started seeing an improvement with IBS symptoms. It works, but you have to give it time and increase the dose until you see the affects.
But don't increase the dose suddenly or you will experience many side effects. In addition to less diarrhea and upset stomach, I'm also sleeping better and feeling better in general on this drug.