Form is a natural result of repetition in music. As composers we must always work to achieve a balance between new material and repetition. When arranging, this balance is even more crucial so that our compositional goals can shine through the arrangement.
Play through (or listen through) your theme and variations. Are there any sections that sound too repetitive, or have you achieved the balance you’re looking for?
Add melodic, harmonic, and rhythmic embellishments to the “boring” sections.
Remember, a small change in the right place can have a huge impact on the piece.