That's very true, I have finished songs that will never be used by my band, but I cherry-pick from them when I'm stuck.
Nick, are you writing for every instrument, too? In my band, the only thing I don't write are the vocals, even though I've even branched into that, too. But I write all the drums, nearly all bass, and most of the keys. I give whole songs to the band. It takes me a very long time to finish songs recently.
The only advice I can think of is this; before I was writing the kind of songs I
wanted to write, I was writing songs that, although bore some resemblance to what I wanted to do, the bassist wanted written. I mean, this went as far as working out an arrangement of a Backstreet Boys cover that we played live and recorded
I found when I had to write to a brief, with the challenge of doing something more inline with another band member's ideals than my own, that I was more prolific. I'd stick in bits that kept my interest, too, but I'd limit those.
Sorry for the lengthy post. Reading your post got me thinking about my own recent slowdown, and the above has only just occurred to me. I'm forced to admit I was more prolific then. A little depressing, frankly.