If you find yourself stuck, the way forward is usually found within the material you’ve already written.
When given a theme or a motive, there are really only two possibilities:
Repetition is self explanatory. Change can be as simple as a sequence or can become quite abstract (retrograde inverted fragmentation, for instance).
Instead of tying yourself up in knots, try applying basic operations to what you already have. Not only will this likely bust through your block, but it will also enhance organic coherence of your work (constant change is disorienting to the listener–this is undesirable unless its an affect you’re aiming at).