Probably a bad copy. This is the most obvious reason.
Can you post the errors you are getting and your burner make and model. This will help in giving you the right info.
A quick solution could be to try the CloneCD profiles found here. You will need to put these in the CloneCD/Profiles directory.
The ones you are intrested in are the SafeDisc ones.
SafeDisc 2 [Amplify].ccp
Custom Profile: required for one-sheep writers in SD2.51+ and no-sheep writers for < 2.51
SafeDisc 2 [Emulate].ccp
Custom Profile: works with just about any writer
SafeDisc 2 [Ignore].ccp
Custom Profile: required for one-sheep writers < SD2.51+ and two-sheep writers for 2.51+
-sheep writers < SD2.51+ and two-sheep writers for 2.51+
Which one you use depends on your CD Writer. So doing this blind try Emulate, Amplify then Ignore.
Note on copy protection.
The Sims: Unleashed:- SafeDisc 2.80.010.