How long are you freezing your solvent for? You may be correct, in that your freezer isn't as cold as it could be. It may be worth trying a freeze for a few days, maybe evaporating a bit more could help.

It is better to try and freeze precip as much product as possible. A slower precipitation or evap has the chance to leave more goo, though this does depend on the naphtha you're using. I would suggest a longer freeze, on as low a setting as possible.

Peace

Macre