Jim it may well have happened. What is missing is proof of what happened and who caused it to happen. There are many different groups fighting in Syria including illegal coalition troops. All of them have an agenda to follow and truth is generally the first thing to suffer along with many innocent civilians.
Where was the outcry when chemical weapons were used during the Iran/Iraq war, and who was turning a blind eye, knowing full well it was occurring?
It would be the height of stupidity for Assad to use chemical weapons against his own people and that to me makes the scenario that he is the guilty party seem most unlikely. However it suits many factions to make it appear Assad or his Russian allies are guilty.