Does itunes library has limit?

I’ve subscribed for 3 months... Lately, i cant add more music on my iTunes. What happens is immediately i add an album to my iphone6’ iTunes library then after few seconds it dissaperes from my library and shows as if I did not add it before. I have tried different artists but still the same. I even tried to delete some other albums to create some space but that could not work. What could be the problem? Pls help.