Norman,
Take the time to look closely at the handbrake levers on the calipers...
For proper operation it is ESSENTIAL that the levers are not partially seized, and return FULLY and easily to the backstop position when the handbrake is released. If they don't the self-adjusting mechanism won't work correctly and there won't be enough effort applied to the handbrake mechanism no matter how tight you make the cable.
This is a common problem, where the cable gets tightened, on the grounds it will make the handbrake work better - in fact it has quite the opposite effect.. the levers get seized easily then don't return fully, and you'll have no chance of it working as it should. Unfortunately the only surefire way of freeing the levers off sufficiently if they are more than slightly tight, is to dismantle the caliper