I was having the same problem until I was turned on to a nice solution here. I've done this to all my Ibani and am thrilled with the results:
Get a small piece of very thin plastic, like from one of those cheap sandwich baggies or a D'Addario-style string wrapper. Rip it down until it's about the size of a quarter. Place the plastic over the bar receptacle and insert the bar, pushing the plastic down into the holder. That will stiffen it right up.
Also note that your bar holder should be tight, otherwise your flutters will sound more like clunkers.
Fixing a loose arm holder