Why are my cheese curds not well-knitted after pressing?
Several possible reasons...but without seeing the recipe, your make notes, pH markers it is rather hard to say. 
However, I would say that most likely would be too much moisture left in curds due to:


initial curds cut too large
too much rennet, meaning that more moisture was locked into the curd, resulting in the cut being made later than it should be (did you use the flocculation method to determine when to cut the curd?)
pH was too high at various periods during your make, resulting in a "sweeter" curd
too much time between addition of rennet and the cut of the curd (see 2...related)
lack of or insufficient cooking of the curds prior to cheddaring, resulting in too much whey (moisture) left in the curds.
insufficient salting
insufficient pressing (unlikely, since the previous 6 control moisture much more than the pressing)  


Any, some or all of the above.