When developing portrait drawing skills, it is important to learn to see the facial features as objectively as possible. It can be difficult to set aside the symbols we have learned, and the associations we make with certain features, in order to observe them accurately. Drawing from a plaster cast can help you to see the form more objectively, but the symbols can still creep in.
A piece of advice that I often give students who are struggling to separate the symbols (or prior drawing habits) from what they see, is to pretend the cast is an abstract sculpture. Imagine that you decided to observe and draw this form while having no idea what it was. If it were a purely abstract form, you would have no choice but to rely on shapes and value relationships to depict it. If you can put yourself into that mindset, you may be quite surprised by how much the drawing ends up looking like a mouth.
View my post on drawing the structure of a mouth using cross-contour lines.