Why did my ricotta cheese did not completely melt in the Alfredo?
Some ricotta uses gums and stabilizers (usually cheaper ricottas) whereas good ricottas are made with just milk and a bit of acid and a bit of salt. The stabilizers and gums allow the ricotta to hold water rather than proper draining done for better ricottas. The stabilized ricottas often leave a grainy texture after cooking (since the stabilizers don't hold up to heat). 

Check the ingredients of your old brand versus a new brand. If your old one uses just milk, acid and salt versus your new one using gums and stabilizers, that could make the difference. 

Aside: If you're short on good ricottas, cottage cheese (full fat) often is a good replacement depending on context.