I literally had this same issue.
My original 05 battery is duff.
It shows good voltage, easily powers the headlamp nice and bright.
But wasn't able to spin it over after sitting for a month.
Did a run and bump, got it going.
What I have figured out so far is this.
If used daily, it'll start fine (as long as one gets it going on first start and it's a quick start).
If it sits even some days, the battery can't spin it over.
I did a voltage drop check last night during a start try, yikes, it bombed below 9 volts !
I now have my trickle charge harness hard wired to the battery to last the rest of this season.
Battery on trickle until I get around going for a ride.
Going to get a Shorai before next season.
Next is to figure out the biggest Shorai I can get into the battery box.
I'd appreciate hearing what specific Shorai models others have used, and what the class of fit into the battery box was like.
If it sits