I agree with the first point (covers and title aren't as branded as would be ideal for a series), but not the second, necessarily. I think sales history of my first book in first series is somewhere in the same neighborhood as CAKE. I have about 55-60% the # of sales/borrow-equivalents on Book 2 & 3 as of Book 1. That's probably not as good as sell-through in a "true series" with the same couple, but it's not too bad, especially considering that Book 1 was 99 cents for most of its life (it's now $2.99), and that I didn't have a mailing list at all for the first--3 years? Something like that.
Same thing is true of my Montlake series--Book 2 sales are about 65% of Book 1. Both same price.
By contrast, my one "true series" with the same couple has MUCH lower sell-through, hah, much as it pains me to admit it. But there was a year-plus between Book 1 and Book 2, and like Jill, I intended and ended Book 1 as a standalone at the time. It may have more to do with length of time between publication of Book 1 and Book 2 (my first three books in my first series were published on the same day) as anything else. (Montlake series is different because of course they push the heck out of all the books. But Book 1 has had more price promos of course, and not everybody reads on, amazingly.
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I think you can get pretty good sell-through from Book 1 to Book 2 and onward with standalone books. But they do have to be branded really tightly.
The other BIG plus of standalones is that you can promote any of them. I've promoted the first 6 or 7 of the 10 books in my first series at various times, and they've all given me a good bump. That could be something else to consider.
I got a lot of traction early on by doing price promos every 3 months or so. You might consider doing more of that. I'd up the price of Books 2 and 3 also.
Best of luck.