Crumbly texture is, I think, from more acid development in pre-pressing, vat stage, characteristic of cheddar type cheeses. Did your Jack recipe call for washing the curd, or just cooking, draining, and pressing? My Jack's all came out crumbly as well, until I discovered washed-curd recipes that yield a product truer to what I expect from a young Jack (a make similar to Colby). After all, without washing the curd, one is basically making a cheddar minus the cheddaring process (which many Tomme recipes are similar to as well). Without washing the curd, other ways to reduce the acidity are to pre-ripen for less time, cook at higher temps (where the bacteria are not as active), and drain as soon as the curd is properly textured, and dry salt the curd before pressing, also don't press too warm.