Why did my ricotta cheese did not completely melt in the Alfredo? Today I sauteed a quarter cup mushrooms in 3 tbsp butter and added 250ml cream. Everything was set for me to add the ricotta and finish the Alfredo, but it only partly melted. Beyond a certain point, there were some "cheese crumbs" that no amount of heat could dissolve, thus robbing me of a velvety textured Alfredo.

I have successfully made Alfredo with ricotta cheese before. Is there any reason as to why the ricotta did not completely melt? The ricotta package was only opened a few days, so it couldn't have gone bad.
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.