MatttyC, it's likely that one is now stretched and will need replacing, or your gas will end up leaking out as soon as you pierce it. I've had similar issues with the O-rings, it seems a bit hit and miss as to whether or not you can reuse them without replacing the O-Rings. This seems to vary between units too from what I can see, as some people have reported issues with this, and others have had none.
Do yourself a favour, and mod it to run an external reg/gas cylinder/bulbs. They are real easy to pull apart and just as easy to put back together. It didnt take me long to get frustrated with this, and have since hooked up a sodastream cylinder and keg king reg. Im thinking this cylinder is prob going to last me a while too...ive done 5 or 6 bottles on it so far and the cylinder pressure gauge has not moved from its original position.
This is my TK unit, the external bottle makes life so much easier, especially when changing between differenty beer styles, you can dial in your pouring pressure to suit. It's also much easier to get replacement cylinders too, most woolies/coles/big w/Kmart that retail sodastream products will have them: