Well, if you got stretch then your milk is probably fine and hasn't been overheated in processing. Possibly you are stirring too much after adding your rennet, if it is already coagulating then you are cutting the curd by stirring too much. You also may just need to wait a little longer if you aren't getting a clean break right away. After I cut the curd, I let it sit for about ten minutes before I stir to allow the curds time to set a little more, I don't know if this is essential or just voodoo on my part. The acidity level is important, if the acid level is too low you won't get good stretch, but if you are using a kit with citric acid and not a culture, it is probably pretty well regulated. Sometimes it just comes down to differences between milk sources (fat, protein, etc.) and the age of the millk. In Rhode Island I use Rhody Fresh and the closer it gets to the use by date the less robust the results I get.
Someone who is more of a pro at this than me may have more ideas.