Page 128: 'Spells cast at a range of Touch require the sorcerer to make a successful Unarmed attack on a resisting target.'
So if a target is actively trying to avoid being touched, he can use evade, or parry, the sorcerer's attempt.
The easiest way to get round this is to Manipulate the spell's range.
For Divine Magic its a bit more complex as the caster must remain in physical contact throughout the casting, so an Unarmed attack followed by Grip (if the target's resisting) would be necessary.