Continuity mistake: When Ahsoka suggests that Chairman Papanoida's daughters might be being held on the Trade Federation ship blockading Pantora, she refers to it as a planet. "Trespass" established that Pantora is a moon. (00:06:47)
Continuity mistake: When Anakin and Padmé are walking towards her ship while discussing the situation, as happens with surprising frequency, Rex, walking behind them, is missing his kama (the kilt). (00:04:35)
Continuity mistake: When Baron Papanoida walks into Jabba's court, his half-cape is missing. (00:10:24)
Answer: Dooku never accepted Ventress as his apprentice, as far as I know; he just kept her as his devoted personal assassin. In Phantom Menace, it is stated that there can be only two Sith at a time, to prevent a power struggle in the lower ranks. To accept Ventress as an apprentice, Dooku would have violated that rule.