First I must say that I absolutely love this recipe, I have come very close to what I would consider the perfect Mozzarella. I do have a few questions though. When I strain the curds my curd mass is very inconsistent some parts being quite firm and others being mush the firm parts make great cheese but the the mush just either mixes in my hot water and disappears or sinks to the bottom and refuses to stick together with the rest of it. I am using raw jersey milk, my homemade yogurt for starter (this blew my mind at how well that works) and fromase rennet tablets.
I have been away from the forum for a very long time, so I'm sorry I didn't see your question.
My guess is that you are not getting consistent curd size when you cut the curd - the mushy parts (do they look like barely set egg whites?) were probably quite a bit larger than the curds that end up firm.
The way to correct this is to stir the curd gently once it has been cut. Use a skimmer to bring the curds up from the bottom of the kettle as you stir and use a knife to cut the larger curds so they are more consistent in size with the rest of the curd.
If that isn't what you meant - if what you are seeing is kind of a Cream of Wheat consistency, it is most likely because the culture didn't get fully incorporated into the milk. You can correct that by putting your culture into a quart sized jar with about a cup of warmed milk, put a lid on and shake it up until the culture is liquid. Then stir it gently but very thoroughly into the warmed milk in the kettle. That will keep the heavier culture from settling at the bottom.
I hope this helps.