Well, it can be both easier and harder to name a series. Easier because you can develop a naming theme. Harder because...you have to develop a naming theme. What is the overarching theme of your series? Not just the first book, but the whole series?
If it's something like, I don't know, "love", then is there a special word for love that you use even once in the series? For instance, if your protagonist loves someone who will never love them back, you could call it the Redamancy Series. Actually, I think I might use that myself.
Then each title would be related to the series' theme, not clunky, and follow an internal pattern. Like parents naming triplets... Adam, Andy, and Adele. Each name stands alone, but put together, they give a general feeling, in this case: A names with D sounds. You could in turn name your books "Love of Character 1", "Love of Character 2", and "Love of Character 3".
Just very basic examples using the love theme.
It would be helpful to know more about your series.