Coming up with new ideas can often be the best way to clarify the ideas you've already got, since the more ideas you have, the more you're going to see in them the patterns and trains of thought which can then be organized into arguments. Deliberately trying to think up new ideas forces you to examine the mental picture you've already formed of the topic. If you're finding new ideas hard to come up with, you need to ask yourself, as part of your reflective practice whether your mental picture of the topic is holding you back in some way by unnecessarily excluding certain areas.
Ideas flow best when you're relaxed about the thinking process and are in a stimulating environment. Discussing the topic with friends or other students will usually be helpful - you needn't worry about other people "stealing your ideas" as they will almost certainly get different things from you out of the conversation, and in any case you don't have to be original. Another way of getting the ideas flowing is by brainstorming, where you set aside a period of time where you try to think up as many new ideas as possible, and deliberately don't worry about whether they're good or bad, or even logically related - it's worth coming up with a lot of ideas that aren't very useful for the sake of a few good ones. Visual aids such as mindmapping can be useful in a brainstorming session.
If, after you've made a serious effort to come up with new ideas, you find yourself feeling stuck, it may be good idea to do something else for a bit so your ideas get the chance to incubate in your unconscious mind. Just make sure you're carrying a [notebook] for when that great new idea comes to you.