Keep trying and you’ll get there with it. For a while I declared my CP filler a waste of money, but now I couldn't be without it.
The ability to get the beer bottled from a keg with very very low levels of added oxygen means that malt and hop aromas and flavours are preserved. I could never get the same when bottle carbonating, but I also realise now my bottling process wasn't ideal.
The best procedure I've found with the CP filler is:
1. Purge the bottle with CO2 for a few seconds before starting filling
2. Fill slow.
3. Fill until you've got liquid coming out of the pressure relief valve, when you withdraw the filler it'll leave a gap at the top of the bottle anyway. This also stops the beer spurting when you remove it from the filler as you've not got any pressurised gas in the headspace.
4. Leave the bottle attached for a few seconds to bleed off head pressure before removing.
5. Give the bottle a light tap so the empty headspace fills with foam (helps purge out any air) and then cap on that.