You're probably suffering from the fact that deskys are often too busy to give any serious thought to career management - they just need to put names to posts, and will dream up any reson to justify it to you thereafter.
One technique I have seen work before is for you to present a solution to the desky, thus alleviating him/her of the need to think too deeply about your posting. Tends to work better for digital posts, however, rather than just moving as a (relatively) junior shag between flying posts (assuming that is the position you are in). However, it's worth doing a bit of investigation into potentially vacant slots. Good luck!
