Jack cheese is a washed curd, so there is less lactose. This can mean less acidity, which (in combination with moisture) will cause the cheese to slump. As a comparison, if you make a feta the same shape, it will likely hold its shape due to the higher acidity of the cheese.
There are a couple of reasons for your problem, likely. The first one is not cooking the curds long enough, etc. The second one may actually be pressing too hard. As rsterne says, pressing too hard at the beginning creates a kind of water balloon effect -- you close the rind and there is nowhere for the whey to escape. The result is a very high moisture cheese on the inside. This often referments after salting causing a crumbly acidic cheese.
My advice for this cheese is to age it only for a couple of weeks and then enjoy it. It is unlikely to survive much past that. It will not taste like a jack cheese, but mild spicy cheeses are also good.
Edit: A good example of over pressing a cheese. Quite some time ago Gavin Webber made a tomme. Here is the making video:
https://www.youtube.com/watch?v=kr_vcD9cyg0 and the eating video:
https://www.youtube.com/watch?v=QCdFFZSnhj4 The main problem in this cheese is that he's over pressed it, which caused it to collapse (not quite as dramatically as yours, though). Then when he eats it, you can see how the cheese is over moist, sticks to the knife and is and breaks apart instead of being flexible. That's from the cultures continuing to work away at the cultures and over acidifying the cheese.
