From the help center:
When should I edit posts?
[...]
Edits are expected to be substantial and to leave the post better than you found it. [...]
That rule is there for a reason: if you can't give a post a substantial enough edit or you can't give it a really necessary improvement (which likely is the reason for the non-acceptance), than the post in first place doesn't really need to be edited. Furthermore, as in your last rejected suggested edit, the post might need to be improved further, and this is another possible reason for the rejections.