For weeks I have asked this particular student to play certain notes shorter. We would play through the spot together, making sure she knew what I was asking of her. But then the next time she played, the notes would no longer be short and separated.
It occurred to me that this might be one of those instances where I needed to come up with a new way of saying what I was trying to say. So instead of again saying, "play those notes shorter", I tried, "listen to the space between the notes". I was pleasantly surprised by the results and the difference it seemed to make in her playing.
Only time will tell if it sticks. (C are you reading this?)
Another example from the same lesson has to do with how we think of something. My student was having trouble with the string crossings. Often she would over extend her arm and hit an extra string. We talked about reaching for the note instead of crossing a string for it.
I came up with an analogy which I'll share here. Say you are headed up stairs and you drop something on the floor right at the bottom of the stairs. There is more than one way to pick it up. Assuming you have only gone up one stair, you can easily reach down to pick up the item - you don't have to physically step down to get it. Same with playing a note on another string. You don't have to physically change the position of your bow arm to play a note on another string - you can reach with your wrist.
I'm sure most of us know this, but I think it helped to talk about it with my student. I know when my teachers took the time to talk things through with me, it made all the difference.