Here's why:
* Noun: A person who is suspected of a crime or wrongdoing.
* Adjective: Doubtful, questionable, or likely to be wrong or inaccurate.
There is no common suffix that forms words related to "suspect" in the way that "-able," "-ing," or "-ed" do.
Perhaps you are thinking of a different word? If you can provide more context, I might be able to help you better understand the word you are looking for.