In my experience the critical point with Ricotta is getting the temp high enough. Once it curdles it should be good to go. If you are using the whey from Feta you might be dealing with inconsistent acid levels. If you don't get curd at 195*(or a little higher because of your altitude?) you might try adding a little citric acid, 1 teaspoon per gallon of milk, or vinegar, 1/4c per gallon of milk.
Christy
Christy