If it's rubbery, that means the curds are bonding well and are retaining too much calcium. Have you ever made an alpine type cheese? Like a swiss? Those wheels to me seem ultra rubbery, they're so pliable. That's because they're cut and pressed before the culture has had a chance to acidify the milk. If you wait and let the milk ripen longer before adding rennet, you will hit a lower pH at drain time, and that should give you a less rubbery, softer curd. Another thing you could do if pH is still high is to settle the curds, but don't stir them too much so they don't loose a lot of moisture. If they settle and rest, the acidity will build up, and the final curd will not be as rubbery.
Could be overcooked or too much rennet, but IMHO, I think it's acidity at the point when the milk is rennetted and then also when fused.
