Some of the newly risen ヴァンパイア can't change into their human face immediatley (I think that's in "conversations with dead people", season 7, Buffy says it)... it could also be that ther's もっと見る demon inside them, cause it's mostly evil ones like the master in season 1.
It also had a lot to do with the shows budget. A Vamp that morphs between human face and Vamp face means hours added in and out of the make-up chair where-as if the Vamp doesn't morph= less time,less money. This is also why when a Vamp gets "dusted" all their cloths turn to dust as well...no fuss= no extra expense.
As far as why most ヴァンパイア don't assume a vamp face most of the time- The character of the Master was designed to be in vamp face permanently to highlight his age and make him appear もっと見る animalistic; make-up artist John Vulich based the Master's appearance on a bat, reasoning that the character has devolved to a もっと見る primal, demonic state over the years. As other 回答 on here also stated, it is also very expensive do this transformation, and Buffy started off with a very low budget. Throughout the series あなた can see the changes in special effects and costumes, as the 表示する grew a bigger following and with that, もっと見る money.
Unlike other vampires, the Master does not assume human appearance. In one エンジェル episode flashback, Darla tells Angelus (Angel) that "the Master has grown past the curse of human features." He bears a strong resemblance to Count Orlok of the film "Nosferatu." Joss Whedon's script for the pilot ("Welcome to the Hellmouth"; the first episode of Buffy) indicates that the character's actual name is Heinrich Joseph Nest and is over 600 years old; however, his appearance in the episode Darla suggests that he's far older, as he already had a demonic appearance in the 17th and 18th centuries.
