Possible culprits are: mixture too wet, mixture packed too loose, temperature too high or humidity too low. I've had that problem twice. Once on my very first batch, which I tried to make in the summer with no curing chamber, and too high a temperature and fluctuating humidity (and loosely packed), and again recently when I made a pair of sopressati, in which the mixture was too wet ( I noticed some sag when I hung them. The top end was pointed and the bottom end was flat. I had also forgotten to put the starter in a little water to dissolve, so I just dumped the starter powder in the mix. The first time was mostly environmental, but the second time my chamber was behaving perfectly, so it had to be one of the things I did. My theory on wetness is that if there is too much water in the mix, it moves out of the center too fast, leaving a hollow, or the water just settles to the bottom of the link early in the drying, leaving loosely packed meat above. The other possibility is that the starter did not fully disperse, leaving parts of the sausage inactive, although I'm not sure how that would leave a hollow center.