Why is my toilet flushing slowly and not emptying the waste from the bowl?
It sounds like the clog is far enough down so that, given enough time to let the water drain out, there is enough room for one flush to enter the pipes fully before hitting the clog but not for any subsequent flushes. 

Like Nate suggested in the comments, your best bet is to snake it. Here is a how to: https://www.wikihow.com/Snake-a-Toilet