I am having trouble transitioning from sketching photos to live models
The big difference is that when you draw from a photo the image is already flattened onto a 2 dimensional plane, however when drawing from life you have to deal with translating 3D to 2D yourself. 

The way we see in 3 dimensions is rather more complex than you might imagine and the way we understand and visualise an object is very different from a photograph. When you first start life drawing there is a conflict between what you 'know' to be there and how to represent that on a flat surface. 

This is especially true for portraits as most humans are innately good at automatically recognising faces but in the context of drawing this can get in the way of seeing the detail and structure of what is actually there because the recognition is an unconscious process. 

There are a number of ways to approach this. 

For drawing figures it can help a lot to have some understanding of anatomy, this will help you break down the complexities of the human form into their underlying structures of bone and muscle a particular pose can be easier to interpret and helps to start to see faces as a form. 

Similarly practicing drawing 'standard' proportioned faces can give you a reference point to see how a specific face differs from an idealised one. 

It can also help to practice drawing complex structures which are less familiar or even totally unrecognizable. This means that you no longer have your expectations of what is there to fall back on and you are forced to look very carefully at the subject. Things like piles of cloth, crumpled paper, driftwood, piles of stones are all common exercises. The point is that because there is no regular structure to them to fall back on you are forced to get all of your information by looking at the subject. 

Also drawing portraits with extreme lighting can help. 

In life drawing practice the real skill is not so much the actual technicalities of drawing as the ability to look properly at a scene and consciously identify its component parts and how they relate to each other. This is a very different skill from copying an existing image.