you're saying it rides OK if you push and bumpstart? but it dies when you're not moving? that's clutches not disengagine, either one, or you've got things pinched and binding up
if it just dies from lack of power when the wheel is down, that can be anything but on a hobbit my first checks would be reeds, compression (ring snapped on install, decomp on derbi head, upside-down piston) or seals - sometimes the inner spring popped out on install, or theyre installed too deep, or the inner lip is prolapsed out. there's that one variator side seal that people mess up, but you specifically mention 17 & 15 seals so I'm trusting that's not your problem