I've definitely had this problem before. I've got half a dozen unfinished things, some from over a decade ago now, that got to varying lengths between 10k and 40k before they sort of petered out and then I was just sat there saying, "well.... now what?"
They all had a plot stretching out ahead of me and an 'endgame', for lack of a better term, but the path to get there felt lifeless and drab. And if
I thought it was drab, I can only imagine how dull it would have been for a reader
I think it's only now in the last year or two that I've finally realised that
my problem (it's probably different for everyone) was that I was writing a about events, rather than about characters living through those events.
(Saying that, in the last two months I've made little headway now I'm right at the end... but that's a whole different problem
)