Juliet I used to have a Pfaff Expression 2 which I think came after your model but I could never get it to do FMQ well. I used both it's darning foot and a one I had for the Pfaff Select 4 and while I could get them to work it was never good enough. I would get skipped stitches or thread breaking. I disliked it so much that I replaced it for a Janome Horizon and I have never regretted it. I think that is probably the one you tried at Houston - it's a dream for FMQ.
But just to be sure, here is a photo of my Pfaff with the darning foot on and the presser foot lever up. Did you have the clear plastic thingy over the needle screw (in the photo I am pointing to it with a chop stick.)?
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And here's a photo of the foot with the lever in the 'half'way' position for FMQ. There is about 1/2 a chopctick width under the foot.
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If the only time you tried FMQ was at Houston on a Janome the the comparison is like driving an automatic Rolls Royce and a good family car.
The only other thought that strikes me is that Patsy Thompson uses Pfaff machines and her classes in FMQ are here in the TQS classroom.
Good luck.