Do you have a serger? I took a lesson with mine not to long ago and I was using tulle and we put it through and presto it gather up nicely.

Chiffon is way too flimsy for a ruffler attachment. I would do a small zig zag over a nylon cord and gather that way instead.

there should be an adjustment on your ruffler that will allow a little more pressure, however usually that type of fabric just isn't firm enough for the ruffler itself.

Monica craftercritter & corinney both have excellent suggestions. Chiffon is a fabric I have never been successful with in a ruffler. Lots of times i've had to do it by hand but if it's a lot of ruffles I would try one of their methods./Lillian

I use left over wash-away stabilizer bits dissolved in a little water to stiffen chiffon, (soak and allow to dry then cool iron).This allows super embroidery on the stiff chiffon. I don't have a ruffle foot but you could try this, because it behaves better under the usual sewing machine foot even for zigzag.
Happy stitching
From Bev

I've done this by running a stitch down the cut edge, then running that stitched line in the center of the ruffler tucker-part (if that made sense). The stitched line gave the tucker-part something to grab onto, and because it was stitched together, both edges ruffled.
Hope this helps.... I love my ruffler...it's older than me (and I'm pretty old), but it works like a champ.