802.11r is the best protocol for roaming. If you have it on, try turning it off. If you have it off, trying turning it on. On the whole, you want it on when you have mulitple access points.
Next, when you tested your device was it using 2.4Ghz or 5Ghz? Does the MC9500 use the same frequency as your device? If they are different then your test results are not valid. You'll need to repeat them using the same frequency.
For devices that are not roaming friendly you can sometimes give them a "hand" by increasing the minimum connect speed. For example, I frequently make the minimum connect speed 12 Mb/s (which disables the old 802.11b). When a device can't sustain this speed it has to disconnect and reconnect to the next nearest AP. This often solves the problem of devices hanging onto an AP when it is a long distance away and there is a closer one.
I often disable the 2.4Ghz band if I can. 2.4Ghz is much more narrow. There are only three non-overlapping channels. So when you have 10 AP's you are guaranteed that some of the APs will be re-using spectrum. Note that 2.4Ghz does have better range - if you can get some good clear spectrum.
Some lower cost devices or older devices don't support 5Ghz so sometimes you don't have a choice.