There are some of the boss characters, Josephina and Moriendor especially, that resist the freeze spell. Any critter with high spiritual resistance will also be less affected by the frost spell when you cast it.

However, a weapon with frost will freeze both, in my experience, as a weapon with frost seems to ignore any resistances from any character. I don't want to say frost weapons are broken but they seem to be the exception when fighting the bosses. I look for a sword with frost and a low recoup time but there are others like frost axes etc. (see attached image) The axes have a longer recoup time so you get less strikes but with frost even they will wear down a boss. I look for frost weapons constantly during game play, replacing a weaker frost weapon with a stronger frost weapon having a frost weapon in my inventory always.

If your character is a Warrior, again with bosses such as Josephina and Moriendor especially, I suggest a frost weapon and using the warrior's special move while invisible. The warrior can attack while invisible by right clicking (making the Special Move the active skill to use it )which is very helpful against the bosses. Josephina and Moriendor tend to blind you and move around a lot so being invisible and using a frost weapon gives you an advantage. One left click will make you visible and remove your advantage so be sure to only right click. Use a Shadows potion to become invisible. The Survivor and Mage cannot use this method as their special moves are different.

Reading your description, It sounds like you may have some slight graphics problems. If your OS is Vista / Win 7 make sure you have right clicked the game shortcut, clicked Properties, the Compatibility tab and check marked "Run this program as an administrator". Also check "Disable desktop composition" and "Disable visual themes". Doing these steps can sometimes help with graphic glitches when running DD on Vista or Win 7. . . =)