Starting almost new lawn mower?

We are now onto our second new Briggs and Stratton lawn mower this season. #1 was started ok the first weekend, and then got progressively worse until a month later when it would not start at all. #2 is now three weeks old and behaving the same way. We got it to run very roughly by liberally spraying it with engine starte fluid, but it does not run well enough to mow the lawn. The fuel is less than 30 days old, the spark plug is ok. Can anyone please tell me what is going on. I had expected a brand new mower not to give us any problems at all. The second mower is a Craftsman model # 917.388830.
Thanks
Thanks