Here are some tips:

You may not get a curd due to your milk being too hot.  Once the milk reaches target temp (90 for queso fresco), you need to remove it from your heat source and sit it somewhere else.  A liquid like milk retains heat for a long time.  With a mesophillic culture, it will kill the culture if it gets too warm so temperature is important.  Be sure to use your thermometer and remove it from any heat source right away while the culture and rennet is working. 

[Just a note here - direct heat on the stove is really not the best way to make cheese as it is very difficult to control the temperature.  You should use a "double boiler" type set-up in which your pot w/milk is setting inside a larger container with water in it.  It is easiest to accomplish by setting your pot of milk in the sink and either running hot tap water into it the sink to bring the temperature or if your tap water isn't hot enough, keep a kettle or pot of water hot on your stove to add to the water in your sink when it gets cooled down.  As the water in the sink cools, drain it out and start with new hot water.]

Be sure to test your thermometer to be sure it is reading correctly.  The ones from NEC are callibrate-able with the screw nut.  If you use the thermometer with the 12" stem, the pack it came in gives directions on how to calibrate.  If your temp is too hot OR too cold, your rennet won't set up as it should.

Ricki Carrol's recipe doesn't state the wait time but most other recipes do.  There are many errors and things just plain left out of her recipes in the book which make the book a little frustrating for someone with no experience.  The wait time allows the culture to begin working and start producing the lactic acid that you need to get the break you're looking for.

Miss do-mess….  are you using raw or pasteurized milk?  That will also make a difference.  I'm answering these questions as if RAW milk.  Let me know if pasteurized and I'll write a bit more on that issue.

2.  After adding the rennet, you want that pot off the heat source.  (As stated above, if you leave it on the heat source, the temperature WILL creep up and will cause several problems including "killing" the culture as mesophillic is not for high temperatures.)  It is okay if the temp. drops a degree or 2 during this period of time.  Don't be alarmed if that happens.  Usually it will stay within a degree as the liquid keeps heat very well.

3.  It is very important to bring the temperature up no more than 2 degrees in a 5 min. period.  I find it much easier to control that by setting my cheese pot into the sink that I have run hot water into.  If the temperature is raising too quickly, simply pull the pot up onto the counter and lkeep it out of the water for a bit. 

The reason for slowly bringing the temp up is that if it is too quick, the curdes "seal" – they get a skin on them – that keeps the whey from escaping.  You want the curds to continue to release whey during this time or it will not release whey properly while pressing, usually causing a "sour" cheese and other problems.  It is better not to get quite to temperature than to get there too quickly. 

As far as your 15 min., sometimes they don't seal and sometimes they do.  Depends on the specific batch and just how quickly things were moving.  You'll just have to try it and see  how it presses.

5.  You mentioned the temperatures of between 100 and 95 in the pot while it was resting after salting.  If it was at all above 95 you are going to have problems with this cheese in the press.  As soon as you have brought brought it up to 95 during the "cooking" time, you need to remove it from the heat.  Again, the pot will retain enough heat to keep things where they need to be.  If you had it on heat during that waiting period I can almost guarantee that they got too hot, again sealing the curd in that situation. 

So, the procedure is: once you have drained the whey and are putting the pot back to sit at the target temp, DO NOT RETURN IT TO A STOVE or hot heat source.  You could set the pot into a sink in which there is warm water – about 95 degrees at this point – and let it rest.  Or, not in the sink at all.  The pot will keep things close enough to temp.  Again, in this stage, if it loses a little heat it is better than getting hotter.

All that being said, when I've had a queso fresco (or other cheese) not turn out so well, we've never had to throw one away.  When I haven't liked the end flavor, we've just grated it and used it for lasagna, pizza, etc., like you would use mozzerella.  I just used those early trials as the opportunity to learn and continued to work out the bugs on the next batches until I got pretty consistent at it!

One trick I learned is that if you let your queso fresco "age" in your regular cold fridge for a week before eating it actually developes a better flavor.  I put mine into a glass container with a tight fitting lid (like the pyrex storage bowls) and don't cut it until a week down the road.  During that week I turn it over morning and evening just like I would if I were doing an aged cheese so that it ripens evenly.

And…don't give up!!!! The more you work with cheesemaking, the more you get the "feel" of things and understand what's going on in the process, it will all make better sense to you and you'll have some great cheeses for your effort! 

PS:  Just realized one of the temperatures I stated in the first post was the goat milk temperature… sorry if I caused confusion there.