Your subpoenas will be ignored if the employees are in Missouri. A person you request the attendance of at trial through subpoena only has to appear if he is a resident of your state. Second, and I'm not 100% sure on this one, but I don't believe serving a subpoena requesting the personal attendance of a specific person is effective when served on the corporate agent. I believe that has to be personally served to either that witness or his/her attorney.
You are in small claims court with an out-of-state corporation. I assume you have filed a complaint. If the corporation does not answer your complaint, then you will get a default judgment which means the corporation cannot dispute they caused whatever injury or harm you are claiming. If they have answered your complaint, my advice would be to serve discovery (interrogatories, requests for production, and requests for admissions) on the attorney who answered the complaint (if there is one) or the corporate agent you have already found. You can find templates for the discovery requests and you can basically ask them to turn over or admit to whatever you want. Requests for Admissions can be particularly effective on an unrepresented opponent because if you say something like "Please admit you were negligent by leaving grape juice on the store floor for six hours which caused me to slip and break my back" and they don't answer within the required time - then the court treats that unanswered RFA as admitted.
Just throwing out things you can do.