Without a picture or more details it is hard to be sure what happened. But a few guesses are: 1) maybe your 1% TSA plates were very dry and absorbed too much moisture from the soft agar. 2) Maybe the bottle of soft agar was not well mixed before you used it so the soft agar layer was not really 0.75% (I've had this happen often), 3) maybe your incubator has very low humidity a0nd dried the plates out too much.