yup.. I bought the 17 round AR Stoner Mag. I had the exact same problem. I have several 20 round .223 mags one of which had a broken follower. So i took the spring out of that one and put in the 6.8 AR stoner mag. With the different spring it worked better, but still won't keep the bolt back after the last round has been fired...Guess we got what we paid for..
I also have a couple AR Stoner .223 30 rd mags and I have never had a problem with them at all..