Big fan of the Stebel compact horns. This dual tone does 139db and comes complete with nuclear warning stickers.
If one of those don't blast you out of your boots, good guess is a bad ground or weak connection. Yes, you can trigger the horn with the stock switch, but a relay wired directly to ground is necessary for full power. Certainly runs rings out of the stock
meep meep on most bikes.
PS - little tip. If the horn goes flup flup when you push the button, you've got it wired up backwards - swap the terminals to take it from suck to blow ...