Having not been there, its very hard to say. Even had I been there, its still somewhat of a guess.

With that in mind, I'd guess that bird was a subordinate bird, and was scared to come over upon seeing your decoys. A subordinate bird will often gobble when far away, then slip up without strutting or gobbling to make sure there isn't a bad boy with that hen he's been hearing.

Your description roughly matches up with that.

Hal