Given the background description of some cooking related query, summarize the question into a title

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.
Why did my ricotta cheese did not completely melt in the Alfredo?