Especially newer improvisers, they are very timid about getting into a scene with more experienced students, because they feel they can't measure up, or that they won't know what to do, or that they - god forbid - won't be funny.
As a result, they tend to overthink their improvisation. They try too hard to be funny, or they try to fill with too much dialogue, or they throw in huge, outrageous character actions that don't seem to fit the scene.
What they're missing is that the most important part of being in a scene is just that - being in the scene. Listening to and observing what's going on will often present very logical means of entry - mention of another character, the appearance of a problem in need of a solution, one character's action just begging for another's reaction...
Occasionally, your entry into the scene may not even be an active one - a scene taking place in a grocery store can easily have a silent shopper in the background. That player, if they're present in the scene, listening and observing, can find the spot to add their one line, their observation, their insertion into the primary scene unfolding. Even if they don't, they've made the scene that much richer by just being supporting characters adding to the environment.
Some of our games which focus on support can help student learn how to just be present.