I usually begin to wind the tool down, to place the spring under tension, then get a big socket that will sit on the retainer and give it a brisk 'tap'. This will free everything up, then when you wind the tool down further, you should be able to pull the valve back up (closed). Wind the tool down just far enough to be able to pull the collets off with a pair of long nosed pliers, or better still, one of those little telescopic magnets. Don't wind it down further than you need to though, you risk pulling the valve into the head.